KYIV, Ukraine--Hundreds of artillery shells exploded along the contact line between Ukrainian soldiers and Russia-backed separatists, and thousands of people evacuated eastern Ukraine, further increasing fears Sunday that the volatile region could spark a Russian invasion.

Western leaders warned that Russia was poised to attack its neighbor, which is surrounded on three sides by about 150,000 Russian soldiers, warplanes and equipment. Russia held nuclear drills Saturday in neighboring Belarus and has ongoing naval drills off the coast in the Black Sea.

The United States and many European countries have alleged for months that Russia is trying to create pretexts to invade. They have threatened massive, immediate sanctions if it does.

Ukrainian President Volodymyr Zelenskyy called on Russian President Vladimir Putin to choose a place to meet where the two leaders could meet to try to resolve the crisis.

“Ukraine will continue to follow only the diplomatic path for the sake of a peaceful settlement,” Zelenskyy said Saturday at the Munich Security Conference. There was no immediate response from the Kremlin.

Zelenskyy spoke hours after separatist leaders in eastern Ukraine ordered a full military mobilization and sent more civilians to Russia, which has issued about 700,000 passports to residents of the rebel-held territories. Claims that Russian citizens are being endangered might be used as justification for military action.

In new signs of fears that a war could start within days, Germany and Austria told their citizens to leave Ukraine. German air carrier Lufthansa canceled flights to the capital, Kyiv, and to Odesa, a Black Sea port that could be a key target in an invasion.

NATO’s liaison office in Kyiv said it was relocating staff to Brussels and to the western Ukraine city of Lviv.

“They are uncoiling and are now poised to strike, ” U.S. Defense Secretary Lloyd Austin said Saturday of Russia's readiness to launch an attack.

U.S. President Joe Biden said late Friday that based on the latest American intelligence, he was now “convinced” that Putin has decided to invade Ukraine in coming days and assault the capital.

A U.S. military official said an estimated 40% to 50% of those ground forces have moved into attack positions closer to the border. The official, who spoke on condition of anonymity to discuss internal U.S. assessments, said the change has been underway for about a week and does not necessarily mean Putin has settled on an invasion.

Lines of communication between Moscow and the West remain open: the American and Russian defense chiefs spoke Friday. French President Emmanuel Macron scheduled a phone call with Putin on Sunday. U.S. Secretary of State Antony Blinken and Russian Foreign Minister Sergey Lavrov agreed to meet next week.

Immediate worries focused on eastern Ukraine, where Ukrainian forces have been fighting the pro-Russia rebels since 2014 in a conflict that has killed some 14,000 people.

Ukraine and the separatist leaders traded accusations of escalation. Russia on Saturday said at least two shells fired from a government-held part of eastern Ukraine landed across the border, but Ukraine's foreign minister dismissed that claim as “a fake statement.”

Top Ukrainian military officials came under a shelling attack during a tour of the front of the nearly eight-year separatist conflict in eastern Ukraine. The officials fled to a bomb shelter before hustling from the area, according to an Associated Press journalist who was on the tour.

Elsewhere on the front lines, Ukrainian soldiers said they were under orders not to return fire. Zahar Leshushun, peering into the distance with a periscope, had followed the news all day from a trench where he is posted near the town of Zolote.

“Right now, we don’t respond to their fire because ..." the soldier started to explain before being interrupted by the sound of an incoming shell. "Oh! They are shooting at us now. They are aiming at the command post.”

Sporadic violence has broken out for years along the line separating Ukrainian forces from the Russia-backed separatists, but the spike seen in recent days is orders of magnitude higher than anything recently recorded by international monitors: nearly 1,500 explosions recorded in 24 hours.

Ursula von der Leyen, the president of the EU’s Executive Commission, said Russian access to financial markets and high-tech goods would be sharply limited under Western sanctions being prepared in case of a Russian attack.

“The Kremlin’s dangerous thinking, which comes straight out of a dark past, may cost Russia a prosperous future,” von der Leyen said.

Denis Pushilin, the head of the pro-Russia separatist government in Ukraine’s Donetsk region, cited an “immediate threat of aggression” from Ukrainian forces in his announcement of a call to arms. Ukrainian officials vehemently denied having plans to take rebel-controlled areas by force.

“I appeal to all the men in the republic who can hold weapons to defend their families, their children, wives, mothers,” Pushilin said. ”Together we will achieve the coveted victory that we all need.”

A similar statement followed from his counterpart in the Luhansk region. On Friday, the rebels began evacuating civilians to Russia with an announcement that appeared to be part of their and Moscow’s efforts to paint Ukraine as the aggressor.

Metadata from two videos posted by the separatists announcing the evacuation of civilians to Russia show that the files were created two days ago, the AP confirmed. U.S. authorities have alleged that the Kremlin’s effort to come up with an invasion pretext could include staged, prerecorded videos.

Ukraine’s military said two of its soldiers died in firing from the separatist side on Saturday.

Authorities in Russia's Rostov region, which borders eastern Ukraine, declared a state of emergency because of the influx of evacuees. Media reports on Saturday described chaos at some of the camps assigned to accommodate them. The reports said there were long lines of buses and hundreds of people waiting in the cold for hours on end to be housed without access to food or bathrooms.

Putin ordered the Russian government to offer 10,000 rubles (about $130) to each evacuee, an amount equivalent to about half of an average monthly salary in eastern Ukraine.

The separatist regions of Ukraine, like much of the country’s east, are majority Russian speaking, and Putin on Tuesday repeated allegations of a “genocide” there in explaining the need to protect them.

One of the evacuees, a Donetsk resident who identified himself only as Vyacheslav, blamed Ukraine's government for his plight.

“Let them calm down," he said. "It’s our fault we don’t want to speak Ukrainian, is that it?”