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 Odessa, 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.

Meanwhile, the United States and Britain would seek to cut off Russian companies' access to U.S. dollars and British pounds if the Kremlin orders an invasion of Ukraine, British Prime Minister Boris Johnson told the BBC.

"The plan that we are seeing is for something that could be really the biggest war in Europe since 1945 just in terms of sheer scale," Johnson said.

Johnson said that sanctions on Russia in the event of an invasion would go much further than previously suggested in public.

He said the United Kingdom and the United States would stop Russian companies "trading in pounds and dollars" – a move that he said would "hit very, very hard" with its impact, the BBC reported.

Britain, home to the center of global foreign exchange trading, had threatened to block Russian companies from raising capital in London and to expose property and company ownership if Russia invades Ukraine.

Given Russia's position as one of the world's top exporters of oil, gas and metals – which are largely priced and settled in U.S. dollars – blocking Russian companies from access to dollar markets could have a stinging impact.

Putin has repeatedly called for reducing reliance on the U.S. dollar trade.

Russia's largest oil company Rosneft fully switched the currency of its contracts to euros from U.S. dollars to shield its transactions from U.S. sanctions, CEO Igor Sechin said in 2019.

Johnson has said the government would target Russian banks and Russian companies. Britain has not spelled out who would fall under the sanctions, but has pledged that there would be nowhere for Russian oligarchs to hide.

Regarding U.S. and Britain’s approach, Russia's first deputy permanent representative to the United Nations said on Sunday that the assessments of U.S. and British spies on Ukraine cannot be trusted as they made so many grave mistakes in the run up to the U.S.-led invasion of Iraq,

"We don't trust the U.S. and British intelligence, they let us down, the whole world, on many occasions enough to remember weapons of mass destruction in Iraq," Dmitry Polyanskiy told Sky.

Polyanskiy said no one should try to tell Russia where it held military exercises on Russian territory.

Amid escalating tensions, Russia says it has taken in tens of thousands of people fleeing the conflict in eastern Ukraine.

According to state news agency Tass, Civil Defence Minister Alexander Chupriyan said 40,000 refugees had arrived in Russia's southern Rostov region and were being accommodated in 92 emergency shelters.

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.”