A new meeting is expected next week in London, and Rubio suggested that could be decisive in determining whether the Trump administration continues its involvement.

Donald Trump has said the US will "take a pass" on brokering further Russia-Ukraine talks if Moscow or Kyiv "make it very difficult" to reach a peace deal. The US president told reporters in the Oval Office on Friday that he was not expecting a truce to happen in "a specific number of days" but he wanted it done "quickly".

The Trump administration made clear it will abandon peace attempts for Ukraine if Russia fails to adhere to any terms, but the president did not say whether there would be consequences.