Ukrainian President Volodymyr Zelenskyy praised American President Joe Biden for a surprise trip to Kyiv on Monday.

Biden made an unannounced visit to Ukraine to meet with Zelenskyy, a gesture of solidarity that comes days before the first anniversary of Russia's invasion of the country.

"Historic. Timely. Brave. I welcomed @POTUS in Kyiv as Russian full-scale aggression approaches its one-year mark. I am thankful to the U.S. for standing with Ukraine and for our strong partnership. We are determined to work together to ensure Ukraine's victory," Zelenskyy tweeted with a video showing moments of Biden's visit.

Newsmax CEO Christopher Ruddy broke the news of Biden's arrival on Twitter shortly after 3 a.m. ET on Monday: "I received a report that Biden has secretly arrived in Kyiv."

During his visit, Biden promised new military aid for Ukraine worth $500 million.

"This visit of the U.S. president to Ukraine, the first for 15 years, is the most important visit in the entire history of Ukraine-U.S. relations," Zelenskyy said.

The Associated Press contributed to this story.

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