The UK has promised to impose sanctions on Russia, which it warned could invade Ukraine in the near future. London condemned President Vladimir Putin’s recognition of two rebellious regions in eastern Ukraine, calling the decision a violation of international law.

British Prime Minister Boris Johnson offered military support in a conversation with Ukrainian President Volodymyr Zelensky, saying a diplomatic solution should be sought to the last but the situation was deteriorating.

“The Prime Minister told President Zelensky that he believes an invasion is indeed possible in the coming hours and days,” Johnson’s office said in a statement following a phone call between the leaders.

“He told President Zelensky that the UK has already developed sanctions against those involved in violating Ukraine’s territorial integrity and that these measures will come into effect tomorrow. The Prime Minister also said that, at the request of the Ukrainian government, he would consider further defensive support for Ukraine.”

Johnson will agree a set of sanctions against Russia at a meeting of the government’s crisis response committee at 09:30 MSC on Tuesday, a Downing Street spokesman said.

Russian President Vladimir Putin on Monday formally recognized the independence of two rebellious regions – the self-proclaimed Donetsk People’s Republic and Luhansk People’s Republic – despite Western warnings that such a move would be illegal and derail peace talks.

“This is … a blatant violation of Ukraine’s sovereignty and integrity,” Johnson said at a news conference ahead of his conversation with Zelensky.

“It’s a repudiation of the Minsk process and the Minsk agreements, and I think it’s a very bad omen and a very dark sign.”

After Putin signed the decree, Johnson told Zelensky that the move made the Minsk process unworkable, a view echoed by Foreign Secretary Liz Truss, who said Russia had violated international law.

“We will not allow Russia’s violation of its international obligations to go unpunished,” Truss said.