U.S. President Joe Biden made an unannounced visit to war-torn Ukraine on Monday, arriving in Kyiv as Washington signals its ongoing support ahead of the one-year anniversary of Russia's invasion.

Biden's visit came ahead of a planned meeting with NATO allies in Poland. He is expected to give a speech at the Royal Castle Arcades in Warsaw on Tuesday evening to offer an appraisal of international support during the first year of the war and to address "how we will continue to stand with the people of Ukraine for as long as it takes," White House spokesperson Karine Jean-Pierre said in a statement earlier this month.

Biden also plans to meet in Poland with leaders of the Bucharest Nine, a group of eastern NATO allies formed in 2015 in response to Russia's annexation of Ukraine's Crimean Peninsula.

Ukrainian President Volodymyr Zelenskyy made a dramatic visit to the United States in December, his first known international trip since the Russian invasion began in February 2022. Zelenskyy met with Biden at the White House in Washington, D.C., before addressing members of U.S. Congress on Capitol Hill.

Ukraine's military intelligence agency has accused Russia of planning to stage "large-scale nuclear exercises" to coincide with President Joe Biden's visit to Europe next week.

The GUR said Russia is preparing for test launches of nuclear capable missiles from land and sea, the agency said in a statement Sunday on its official Telegram channel.

The GUR said a nuclear armed submarine has been placed on the "highest level" of combat readiness and that strategic bombers have been moved to a base in Tambov, Russia.

The agency claimed the exercises are intended disrupt President Joe Biden's European trip.

"Such actions of the military and political leadership of the Russian Federation, in particular, are an attempt to hinder Joe Biden's visit to Europe, which is scheduled for February 20-22, through direct nuclear blackmail and to weaken international support for Ukraine," the GUR statement said.

- ABC News' Patrick Reevell

Vice President Kamala Harris met with the British, Finnish and Swedish prime ministers before departing Munich on Saturday.

Amid concerns in Europe that Republican lawmakers could dampen U.S. aid to Ukraine, the Finnish prime minister, Sanna Marin, thanked Harris for saying U.S. support for Ukraine would persevere domestic political differences.

Both Marin and Swedish prime minister, Ulf Kristersson, spoke with hope about joining NATO. But in recent days, there have been signals from NATO, the Finnish and the Swedish that perhaps they will not join at the same time as they had hoped due to continued Turkish objections over Swedish membership.

-ABC News' Ben Gittleson

National Security Council spokesperson John Kirby provided a preview Friday of President Joe Biden's upcoming trip to Poland, saying the president's main message will be continued support from the United States in the face of Russian aggression.

"On Tuesday evening, local time, President Biden will deliver remarks in Warsaw on how the United States has rallied the world to support the people of Ukraine as they defend their freedom and democracy. President Biden will make it clear that the United States will continue to stand with Ukraine, as you've heard him say many times, for as long as it takes," Kirby said of Biden's major planned address.

"As we approach the one-year mark since this invasion, we can proudly say that our support for Ukraine remains unwavering and our alliances and our international coalition in support of Ukraine remain stronger than ever," he added.

Biden is scheduled to arrive in Poland on Tuesday morning and meet with Polish President Andrzej Duda. On Wednesday, he'll meet with leaders from the so-called Bucharest Nine --Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia -- which are nine NATO countries in Eastern Europe.

Kirby was asked about Biden meeting Ukrainian President Volodymyr Zelenskyy, or traveling anywhere besides Warsaw, like the border town of Rzeszow, but he shot down both ideas.

"There is no meeting with President Zelenskyy scheduled for the trip right now," he said. "Right now, the trip is going to be in Warsaw."

-ABC News' Sarah Kolinovsky