The US embassy in Kyiv warned on Friday of a "potentially significant" air attack in the coming days.

"The U.S. Embassy in Kyiv has received information concerning a potentially significant air attack that may occur at any time over the next several days," the embassy said on its website.

"The Embassy, as always, recommends U.S. citizens be prepared to immediately shelter in the event an air alert is announced."

'Very good call with Zelensky'

Earlier, the White House said President Donald Trump had a "very good and productive" call with Ukrainian President Volodymyr Zelenskiy on Thursday, adding that Trump hopes both Ukraine and Russia will agree next week to a proposed 30-day ceasefire.

White House spokeswoman Karoline Leavitt, speaking to reporters at a White House briefing on Friday, also reiterated that a Trump meeting with Russian President Vladimir Putin next week in the Middle East was not going to happen.

In Kyiv, Zelenskiy's chief of staff, Andriy Yermak, said he spoke by telephone on Friday with U.S. Secretary of State Marco Rubio and other officials about the proposed ceasefire as part of moves towards a peace agreement.

"The main focus was the question of the ceasefire and prospects for a peace settlement," Yermak wrote on Telegram, adding that U.S. envoys Steve Witkoff and Keith Kellogg had also taken part. Ukrainian Defence Minister Rustem Umerov was also present.

"Also discussed was the importance of implementing the points on which our presidents agreed," Yermak wrote.

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