Kyiv, Ukraine - US President Joe Biden arrived in Kyiv for a surprise visit as the first anniversary of Russia's full invasion of the country approaches, images from the Ukrainian capital showed on Monday.

Before his planned trip to Poland this week, Biden arrived in Kyiv on Monday, his first visit to the country since the Russian war began on February 24, 2022. He met with his counterpart, Volodymyr Zelensky, with whom he strolled through Ukraine's capital even as air raid alarms were blaring.

"As we approach the anniversary of Russia’s brutal invasion of Ukraine, I'm in Kyiv today to meet with President Zelenskyy and reaffirm our unwavering commitment to Ukraine’s democracy, sovereignty, and territorial integrity," Biden tweeted.

The visit had not been announced for security reasons. Details on timing and means of transportation remain unclear, and the surprise is all the greater for National Security Council spokesperson John Kirby having denied on Sunday that Biden had any plans to enter Ukraine.

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"I thought it was critical that there not be any doubt – none whatsoever – about US support for Ukraine in their brutal attack by Russia," Biden explained, per Bloomberg.

Biden is scheduled to meet with Polish President Andrzej Duda to discuss "collective efforts to support Ukraine and bolster NATO's deterrence," according to the White House.

He will also meet with leaders of Eastern members of NATO and give a speech on "how we will continue to stand with the people of Ukraine for as long as it takes," per the White House.

Because travel from Poland to Ukraine would have required a lengthy train ride or potentially dangerous flight, Biden's advisers had briefed the press that the risks weren't worth it. The fact that president planned to make the trip in reverse seems not have been anticipated.

He will depart Poland on Wednesday, two days before the one-year anniversary of Russia's invasion.