U.S. President Joe Biden said Monday that Russian President Vladimir Putin was "dead wrong" when he started the war in Ukraine on Feb. 24, 2022.

"When Putin launched his invasion nearly one year ago, he thought Ukraine was weak and the West was divided," Biden said in a statement. "He thought he could outlast us. But he was dead wrong."

The White House released the statement from Biden as he made an unannounced visit to the war-torn country, arriving in Kyiv on Monday morning.

"As the world prepares to mark the one-year anniversary of Russia's brutal invasion of Ukraine, I am in Kyiv today to meet with President Zelenskyy and reaffirm our unwavering and unflagging commitment to Ukraine’s democracy, sovereignty, and territorial integrity," Biden said.

"Today, in Kyiv, I am meeting with President Zelenskyy and his team for an extended discussion on our support for Ukraine. I will announce another delivery of critical equipment, including artillery ammunition, anti-armor systems, and air surveillance radars to help protect the Ukrainian people from aerial bombardments," he continued. "And I will share that later this week, we will announce additional sanctions against elites and companies that are trying to evade or backfill Russia's war machine. Over the last year, the United States has built a coalition of nations from the Atlantic to the Pacific to help defend Ukraine with unprecedented military, economic, and humanitarian support -- and that support will endure."

Biden added: "I also look forward to traveling on to Poland to meet President Duda and the leaders of our Eastern Flank Allies, as well as deliver remarks on how the United States will continue to rally the world to support the people of Ukraine and the core values of human rights and dignity in the UN Charter that unite us worldwide."

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

The nuclear watchdog of the United Nations is calling for the immediate establishment of a "nuclear safety and security protection zone" around the Russian-occupied Zaporizhzhia nuclear power plant in southeastern Ukraine.

The recommendation, among several others, was made in a second report released Tuesday by the International Atomic Energy Agency, which deployed an expert mission to the besieged plant last week.

"The IAEA recommends that shelling on site and in its vicinity should be stopped immediately to avoid any further damages to the plant and associated facilities, for the safety of the operating staff and to maintain the physical integrity to support safe and secure operation," the agency wrote in the report. "This requires agreement by all relevant parties to the establishment of a nuclear safety and security protection zone around the ZNPP."

The Zaporizhzhia nuclear power plant is the largest in Ukraine and all of Europe. Invading Russian forces overran the site and the surrounding town of Enerhodar in early March. The Ukrainian workers have been left in place to keep the plant operating, as it supplies electricity across the war-torn country, but the site is now on the front line between Russian-occupied and Ukrainian-controlled territory. Moscow and Kyiv have traded blame for shelling at or near the plant in recent days and weeks, fueling fears that the conflict could spark a radiation disaster.

The IAEA said it aims to maintain a "continued presence" at the plant to "help further improve and deepen the understanding of the situation."

"While the ongoing shelling has not yet triggered a nuclear emergency, it continues to represent a constant threat to nuclear safety and security with potential impact on critical safety functions that may lead to radiological consequences with great safety significance," the agency wrote in the latest report.