'Don't be bullied', US speaker Pelosi tells Zelensky

"Do not be bullied by bullies. If they are making threats, you cannot back down. That's my view of it," she told a news conference in the Polish city of Rzeszow, a day after talks in the Ukrainian capital with President Volodymyr Zelensky.

"We are here for the fight and you cannot fold to a bully."

Her unannounced trip to Kyiv made her the most senior US official to visit the Ukrainian capital to date, following visits by US Secretary of State Antony Blinken and US Defence Secretary Lloyd Austin last month.

"We are visiting you to say thank you for your fight for freedom... Our commitment is to be there for you until the fight is done," Pelosi told Zelensky when they met, footage of which was released by his office.

In a tweet, Zelensky thanked the United States "for helping protect the sovereignty and territorial integrity of our state" and for "leading strong support for Ukraine in its fight against Russian aggression".

Pelosi told the news conference their talks had focused on "security, humanitarian assistance, economic assistance and eventually rebuilding when victory is won".

"We leave with a firmer understanding and more current understanding of what needs to be done, with a deeper appreciation and inspiration from those who are in the fight," she said.

Representative Jason Crow, a military veteran and House Intelligence Committee member, said it was essential the Ukrainians "have what they need to win".

"We are not interested in stalemates, we are not interested in going back to the status quo. The United States of America is in this to win it and with Ukraine until victory is won."