Treasury Secretary Janet Yellen made an unannounced trip to Ukraine on Monday, meeting face-to-face with Ukrainian President Volodymyr Zelenskyy in Kyiv on a visit aimed at reinforcing America's commitment to the war-torn nation following its invasion by Russia.

A Treasury official said the purpose of Yellen's trip is to reaffirm President Joe Biden's message during his visit to Ukraine one week ago, in a show of support for the country's fight against Russia.

Yellen is in the Ukrainian capital for meetings and engagements to discuss the importance of financial assistance to Ukraine from the U.S. and other allies, and will announce America's latest $1.25 billion transfer of economic and budgetary support to the country. 

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The Treasury secretary is also expected to highlight the sweeping sanctions Western nations have imposed on Russia as part of efforts to hinder the country's ability to fund its military.

Following Yellen's sit-down with Zelenskyy, his office issued a statement calling the meeting "important" and expressing gratitude for the ongoing U.S. support.

"The United States has been powerfully supporting us since the first days of this war not only with weapons, but also on the financial front. We really appreciate it," the statement reads. "Thank you for systematic steps to increase sanctions pressure on the aggressor state. It is necessary to further strengthen sanctions to deprive Russia of the ability to finance the war."

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Yellen, speaking to reporters by phone, said the U.S. would also study Ukraine's calls to impose sanctions on Russia's nuclear energy sector, but needed to be "mindful" of potential consequences of such an action on Western allies.

Yellen said the U.S. and its allies were discussing strategies to ensure that Russia pays for the devastation that its war, now in its second year, with estimates in the hundreds of billions of dollars and growing every day.

Washington has confiscated assets used in criminal activity but central bank and other large pools of assets frozen by sanctions are another matter.

"We have on this small scale, seized assets, but there are certainly legal challenges in doing more than that," Yellen said.

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In a separate press release, Zelenskyy's office said the Ukrainian president emphasized the importance of the Biden administration's decision last week to commit $9.9 billion in direct budget support to Ukraine over the course of 2023, saying the funds "allow us to cover the unfunded part of the state budget and support the segments of the population that need it most."

Reuters contributed to this report.