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London • Tense relations between Britain and Russia came under further strain Monday as Kremlin-backed broadcaster RT accused the U.K.'s NatWest bank of shutting down its accounts without explanation.

The editor of the government-funded television channel, Margarita Simonyan, tweeted: "They've closed our accounts in Britain. All our accounts. 'The decision is not subject to review.' Praise be to freedom of speech!"

Russian Foreign Ministry spokeswoman Maria Zakharova posted on Facebook: "Looks like, while leaving the EU, London has left all its freedom of speech obligations there."

RT, formerly known as Russia Today, published what appears to be a letter from NatWest informing the broadcaster that "we have recently undertaken a review of your banking arrangements with us and reached the conclusion that we will no longer provide these facilities." The letter said the firm's bank accounts would be "cancelled and closed" on Dec. 12.

Royal Bank of Scotland Group, which owns NatWest, said such decisions "are not taken lightly" — but indicated it might not be set in stone.

"We are reviewing the situation and are contacting the customer to discuss this further," RBS Group said in a statement. "The bank accounts remain open and are still operative."

Britain's Treasury — which was not told of the bank's decision in advance — declined to comment. Britain has imposed economic sanctions on a number of Russian individuals, but has not introduced any new measures since February 2015.

On Sunday, British Foreign Secretary Boris Johnson said he wanted to increase pressure on Russia over its military campaign in support of Syrian President Bashar Assad.

After meeting with U.S. Secretary of State John Kerry to discuss ways of ending the violence in Syria, Johnson said the West should use the diplomatic tools at its disposal "to make President Putin and the Russians feel the consequences of what they are doing."

RT has been chastised by Britain's broadcast regulator for breaches of the U.K.'s broadcasting code, including a story alleging the BBC staged a chemical weapons attack in Syria.