Great Britain Prime Minister Theresa May said Wednesday that her country is expelling 23 Russian diplomats following an alleged nerve agent attack in the English town of Salisbury last week.

The prime minister said the diplomats have been identified as "undeclared intelligence officers," and that they have one week to leave the country.

The decision ramps up tensions between Moscow and London and may put pressure on the Trump administration to respond in backing one of the closest allies of the United States.

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President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE told May in a phone call on Tuesday that the Kremlin needs to give Great Britain “unambiguous answers” about the attack, and that the U.S. was ready to provide assistance in their investigation.

The White House said the two leaders agreed that whoever carried out the attack needs to face consequences.

Trump told reporters on Tuesday that he believes May's assertion that Moscow was behind the attack.

"It sounds to me like it would be Russia based on all the evidence they have," Trump said. "It sounds to me like they believe it was Russia and I would certainly take that finding as fact."

May told the House of Commons that Great Britain "will fundamentally degrade their [Russia's] intelligence capability in the U.K. for years to come."

May said on Monday it was "highly likely" that Russia was behind the attack, which left Sergei Skripal and his daughter critically ill.

Skripal is a former Russian intelligence officer who has been working for Britain.

British intelligence have labeled the poison as “military grade” and “of a type developed in Russia.”

Three planned visits from Russian Foreign Minister Sergey Lavrov have been canceled, and ministers from Great Britain, along with the royal family, will be absent from the World Cup in Moscow.

Great Britain also announced that it will freeze Russian assets found to be a threat.