British Prime Minister Boris Johnson, in a telephone conversation with Ukrainian President Volodymyr Zelensky, promised to consider a possibility of increasing military assistance to Ukraine, the prime minister's office said.

"The Prime Minister said he would explore sending further defensive support to Ukraine, at the request of the Ukrainian Government," the report said.

Outlining his "grave concern" at recent developments in the region, the Prime Minister told President Zelensky that he believed "an invasion was a real possibility in the coming hours and days."

The report also said the leaders agreed that "the West needed to support Ukraine in the event of an invasion but should continue to pursue a diplomatic solution until the last possible second."

The Prime Minister strongly condemned the Kremlin's decision today to recognise "Luhansk and Donetsk as independent states," and said the move "made the Minsk agreements and process unworkable." He added that the UK was already engaging with partners on the issue and said the UK would raise it at the United Nations Security Council and Organization for Security and Co-operation in Europe in the coming days.

He told President Zelensky that the UK "had already drawn up sanctions to target those complicit in the violation of Ukraine's territorial integrity, and that those measures would come into force tomorrow [on Tuesday]."

The prime minister's office also said that Johnson had scheduled meetings of the COBRA emergency response committee on the situation around Ukraine for Tuesday morning.