The US Treasury Secretary Janet Yellen has paid a surprise visit to Kyiv to reaffirm support for Ukraine, and has committed to providing more than $10 billion in budget support by September.

Yellen met President Volodymyr Zelensky, Finance Minister Serhiy Marchenko and other key government officials on Monday, and reiterated assurances President Joe Biden delivered in Kyiv on Feb 20.

"America will stand with Ukraine as long as it takes," Yellen, flanked by sandbags at the cabinet ministers' office, told Ukraine's Prime Minister Denys Shmyhal in a trip also aimed at shoring up support in the US for continued aid.

In a private meeting with Zelensky late in the afternoon, Yellen commended him "for his leadership and resolve in the face of Russia's illegal and unprovoked war".

She welcomed Zelensky's actions to strengthen governance and tackle corruption, actions she said were needed to ensure that US economic aid is spent responsibly. She reiterated the message when she met Marchenko.

Shmyhal said after talks with Yellen that the country's budget deficit this year is forecast to be $38 billion, and the US is committing to providing more than $10 billion in budget support by September.

A new platform is being introduced to help coordinate the work of the G7 countries and international financial organizations, including the International Monetary Fund and the World Bank, with regard to financial support for Ukraine, Shmyhal said.

He and Yellen also discussed the impact of sanctions on the Russian budget.

"We also discussed the continuation and effectiveness of sanctions on the supply of high-tech items to be used in the military sphere of Russia," Shmyhal said.

A year after the outbreak of hostilities in Ukraine, all of Russia's critically important banks, with the exception of organizations with foreign capital, have in some way been sanctioned. The most recent sanctions include restrictions on three Russian financial institutions, Alfa-Bank, Tinkoff Bank and Rosbank.

The Russian newspaper Kommersant quoted Artyom Kasumyan, a lawyer, as saying that new restrictions do not look so bad against the backdrop of a year of sanctions pressure.

"The EU discussed the 10th package of sanctions for so long that everyone knew ahead of time who would be sanctioned. Banks had plenty of time to prepare."

Russia's Defense Ministry alleged on Tuesday that the US is planning a provocation in Ukraine using toxic chemicals.

The ministry cited John Sullivan, a former US ambassador to Russia, as saying that "Russian troops plan to use chemical weapons in the special military operation area".

Igor Kirillov, chief of radiation, chemical and biological defense troops of Russia's armed forces, said: "We regard this information as the intention of the United States and their accomplices to carry out a provocation in Ukraine using toxic chemicals."

Russia "will identify and punish the true culprits", he said.

In Finland, a parliamentary debate aimed at accelerating the country's application to join NATO was expected to begin on Tuesday.

That was to coincide with a visit by NATO Secretary General Jens Stoltenberg, who was due to meet Finland's Prime Minister Sanna Marin and its President Sauli Niinisto. A final vote was expected on Wednesday.

Agencies contributed to this story.