MOSCOW — President Biden will speak to Ukraine’s President Volodymyr Zelensky by phone Sunday as Western allies scramble to deter a possible Russian attack on Ukraine, as Moscow intensifies its pressure on NATO allies and Kyiv.

Zelensky’s spokesman Sergii Nykyforov confirmed the Biden-Zelensky call Sunday in a comment on Facebook. The Ukrainian president, seeking to avoid panic and damage to his country’s economy, has played down fears of an imminent Russian invasion and has shown irritation at increasingly dire U.S. warnings.

After the call between President Biden and Russian leader Vladimir Putin on Saturday produced no breakthrough, German Chancellor Olaf Scholz will take his turn at averting a Russian invasion with meetings in Kyiv and Moscow.

Scholz, who has faced criticism that his government is not doing enough to support Ukraine, is due to meet Zelensky in Kyiv on Monday and with Putin in Moscow on Tuesday.

Diplomats and citizens of Western countries were departing Kyiv on Sunday after U.S. warnings that an invasion could happen at any time.

Biden, in an hour-long call with the Russian president, said the United States and its allies would “respond decisively,” as officials warned that Moscow could imminently invade Ukraine, likely starting with a barrage of air or missile strikes.

The grim assessment, unveiled Friday, is based on new intelligence — and evidence on the ground — that Russia, with 130,000 troops and major weaponry surrounding Ukraine on three sides, is now fully prepared to launch an attack.

National Security Adviser Jake Sullivan said Sunday that U.S. officials grew more alarmed over a possible invasion because of an accelerated Russian military build-up on Ukraine’s borders in the past 10 days.

“And what we’ve seen just in the last 10 days or so is an acceleration of that buildup and the movement of Russian forces, of all varieties closer to the border with Ukraine, in a position where they could launch a military action very, very rapidly,” he said on CNN’s “State of the Union" adding that it was still impossible to predict what would happen.

He said an attack would likely begin with “a significant barrage of missiles and bomb attacks" that would likely kill some innocent civilians.

“It would then be followed by an onslaught of a ground force moving across the Ukrainian frontier -- again, where innocent civilians could get caught in the crossfire or get trapped in places that they could not move from," he said.

Moscow’s continued build-up is being accompanied by simultaneous Russian exercises in southern Russia, the Black Sea, and in Belarus, Ukraine’s northern neighbor, where a massive Russian-Belarusian military exercise is underway.

Sullivan said if Russia attacked Ukraine, NATO would respond decisively and ensure that the West emerged from the crisis “stronger, more determined, more purposeful than we have been in 30 years, and that Russia ultimately suffers a significant strategic cost for military action.”

He rejected accusations that the U.S. was spreading panic, saying transparency was intended to deny Russia the opportunity to “spring something on Ukraine or the world.”

As major Russia-Belarus war games continued on Ukraine’s border, the Kremlin complained late Saturday that the United States and NATO were ignoring its main demand for an end to the military alliance’s expansion.

Germany has faced sharp criticism in Ukraine over its moves to block third countries from sending German-manufactured weapons to Ukraine, and Berlin’s reluctance to send Ukraine arms.

Russian Foreign Ministry spokeswoman Maria Zakharova warned Sunday the Ukraine ceasefire monitoring mission of the Organization for Security and Cooperation in Europe was being used as a tool to stoke tensions over Ukraine, after the OSCE informed members that several countries were withdrawing mission staff for security reasons.

She said states were trying to “manipulate” the monitoring mission through “filthy political games.”

The mission monitors ceasefire violations between Ukraine and two Russian-backed separatist regions in eastern Ukraine, as part of a stalled seven year peace agreement that has failed to end the conflict. Marathon talks in Berlin Thursday to try revive the peace deal failed to find a path forward.

"The mission is being deliberately dragged into the militarist psychosis stoked by Washington and being used as an instrument for a possible provocation," Zakharova said in a comment on the Russian Foreign Ministry website Sunday.

“We proceed from the understanding that in the environment of artificially stoked tension, the monitoring activities of the mission in full conformity with its mandate are now in demand as never before,” Zakharova said.

A spokesman for Zelensky’s office said in a statement that there were no plans to close Ukraine’s civilian air space, after a statement on the SkyUp airline website that its flight from Portugal to Kyiv had to land in Chisinau, Moldova, when the aircraft’s owner said the plane was barred from entering Ukraine. KLM suspended flights Saturday.

“The most important point is that Ukraine itself sees no point in closing the sky. This is nonsense​,” Mykhailo Podolyak said, criticizing what he called “maximum media escalation” of the crisis. He said if individual airlines changed their flight schedules, it had nothing to do with the government of Ukraine.

Israel sent messages to citizens in Ukraine to leave the country immediately. Israel’s Foreign Ministry estimates there are some 15,000 Israeli citizens in Ukraine.

U.S. Secretary of State Antony Blinken said there was still a path of diplomacy and de-escalation open to Moscow. “The diplomatic path remains open,” he said. "The way for Moscow to show that it wants to pursue that path is simple. It should de-escalate, rather than escalate,” Blinken said, speaking after meetings in Honolulu.

Ukraine’s foreign minister, Dmytro Kuleba, maintained that the "situation remains under control,” saying in a video statement released by the foreign ministry on Sunday that Ukraine continues to work with partners to settle the crisis diplomatically. “We are prepared for any scenario of development of events,” he said. "We have not been sitting with our arms folded for the last months, we have prepared for all scenarios – absolutely all – and as of now we are ready for them.”

Russia has denied plans to attack and demanded that Ukraine, Russia’s pro-Western neighbor, be permanently barred from joining the NATO, calling this a “red line” for Russian security. NATO has refused to budge on its open-door policy.

Russia says it plans to withdraw its forces from Belarus after major military drills involving Russian and Belarusian forces due to end in a week. Military analysts have warned the exercise could be used as cover for an invasion.

A spokesman for the Belarus opposition, Franak Viacorka, said only part of Russian forces in Belarus were involved in the exercise.

“Some Russian troops were deployed outside the places of drills, near the border with Ukraine in the regime of secrecy. We demand full transparency and their immediate withdrawal from Belarus territory,” he said on Twitter.

Russia’s deputy ambassador to the United Nations called on the United States to provide any evidence that makes it certain of a Russian attack. “Please share this information with us too, since we are not aware of it as well!” Dmitry Polyanskiy tweeted late Saturday.

Kuleba said Saturday he held a string of calls with European counterparts about the security crisis, including Alexander Schallenberg of Austria, with whom he discussed “active efforts by the EU and the wider international coalition to protect Ukraine and deter Russia from further aggression.”

He also tweeted that he had discussed joint diplomatic efforts to reduce tensions in the Black and Azov seas with Turkish foreign minister Mevlüt Çavuşoğlu, after Russian warships entered the Black Sea last week for large-scale naval drills that Ukraine’s Foreign Ministry said would render international shipping lanes “virtually impossible” to navigate.

In the latest wave of European diplomacy, French President Emmanuel Macron spoke with Putin for more than 90 minutes Saturday, his office said, and held separate calls with Zelensky and German Chancellor Olaf Scholz. The German leader is due to meet with Putin in Moscow next week.

Britain’s defense secretary Ben Wallace, who visited Moscow for talks with his Russian counterpart on Friday, said it is “highly likely” that Putin will order an attack on Ukraine. He likened the 11th-hour efforts to reach a diplomatic solution to the crisis to the failed attempts at appeasement in the lead-up to World War II.

“It may be that [Putin] just switches off his tanks and we all go home, but there is a whiff of Munich in the air from some in the West,” he told Britain’s Sunday Times — in an apparent reference to the appeasement policies of the 1930s that allowed Hitler to annex the border area of what was then Czechoslovakia, known as the Sudetenland, but failed to avert a world war.

Despite the heightened warnings that Russia could invade at any time, the European Union said Saturday it is not closing its diplomatic missions in Ukraine. “They remain in Kyiv and continue to operate in support to EU citizens and in cooperation with the Ukrainian authorities,” European Union foreign policy chief Josep Borrell said in a statement.

But Australia on Sunday joined a growing list of governments ordering the evacuation of their embassies in Kyiv as the security situation escalates. The United States, Germany, Britain, Latvia, Norway, the Netherlands, Israel, South Korea and Japan and others have been scaling back their diplomatic presence and urging their citizens to get out of Ukraine as soon as possible in recent days. On Saturday, Canada said it will relocate its staff from Kyiv to Lviv.

“The situation is deteriorating and is reaching a very dangerous stage,” Australian Prime Minister Scott Morrison, who last week hosted Secretary of State Antony Blinken for security talks, said on Sunday. “The autocratic, unilateral actions of Russia, to be threatening and bullying Ukraine, is something that is completely and utterly unacceptable,” he added.

Blinken, speaking to reporters in Hawaii on Saturday on the final leg of his Pacific tour, said that the risk of Russian military action in Ukraine is “high enough and the threat is imminent enough” that evacuating most of the staff at the U.S. Embassy in Kyiv is the “prudent thing to do.”

David L. Stern in Kyiv, Ukraine, contributed to this report.