Washington — Treasury Secretary Janet Yellen made a surprise trip to Ukraine to underscore the U.S. commitment to backing Kyiv as the war with Russia enters its second year, meeting with Ukrainian President Volodymyr Zelenskyy one week after President Biden did the same.

Yellen is visiting the country "to discuss critical economic assistance and our efforts to hold Russia accountable for their illegal and brutal war," Treasury Department spokesperson Lily Adams tweeted, along with a photo of Yellen at a train station.

The Treasury Department is responsible for imposing international sanctions against Russian individuals and entities, as well as delivering economic aid to the country, to the tune of $14 billion since the war began. The Treasury Department said Yellen was announcing the recent transfer of $1.25 billion in economic and budgetary aid while in Kyiv.

Yellen and other U.S. officials met with Zelenskyy and his top aides, with Zelenskyy posting a video to his Telegram channel showing him welcoming the secretary and thanking her for her support.

"I'm in Kyiv to reaffirm our unwavering support of the Ukrainian people," Yellen wrote in an op-ed in the New York Times coinciding with her visit. "Mr. Putin is counting on our global coalition's resolve to wane, which he thinks will give him the upper hand in the war. But he is wrong. As President Biden said here last week, America will stand with Ukraine for as long as it takes."

In her op-ed, Yellen explained that economic aid is not only vital for security assistance, but to help meet the basic needs of the Ukrainian people. That includes helping people who have had to flee their homes, and providing housing and utility subsidies. 

"We cannot allow Ukraine to lose the war for economic reasons when it has shown an ability to succeed on the battlefield," Yellen wrote. "Ukraine's military resistance depends on a government that can function effectively, as well as a stable economy that can help finance defense efforts over the long term. By fortifying the 'home front,' our economic assistance is helping make possible Ukraine's stalwart frontline defense against Russia."

A Treasury Department official said planning has quietly been underway for the trip for weeks, following an invitation from Ukrainian Prime Minister Shmyhal. The secretary traveled into Kyiv with a small group of senior aides on an overnight train. Yellen and top Treasury officials viewed the trip as an opportunity to build on Mr. Biden's visit last week and as a chance to reaffirm the United States' commitment to Ukraine. 

Last week during his surprise trip, Mr. Biden walked the streets of Kyiv with Zelenskyy before delivering a speech in Poland. The Biden administration last week announced additional sanctions against Russia and a new round of aid to the country. 

"One year ago, the world was bracing for the fall of Kyiv. Well, I've just come from a visit to Kyiv, and I can report, Kyiv stands strong. Kyiv stands proud. It stands tall. And most important, it stands free," Mr. Biden said last week to applause from 30,000 onlookers waving Polish, U.S. and Ukrainian flags in front of the Royal Castle in Warsaw.

Sarah Ewall-Wice contributed to this report.

Kathryn Watson

Kathryn Watson is a politics reporter for CBS News Digital based in Washington, D.C.