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Police officers stand by a cordon as a forensics tent remains over a bench at the scene connected to the Sergei Skripal nerve-agent attack on March 13 in Salisbury, England.
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LONDON – President Trump said Russia must provide “unambiguous answers” on how a Russian-developed poison was used in England against an ex-Russian spy.

Trump spoke by telephone Tuesday with British Prime Minister Theresa May, telling her the United States is “with the U.K. all the way.” 

The White House said May and Trump agreed on the need for “consequences” for those who use “heinous weapons in flagrant violation of international norms.”

The Russian Foreign Ministry’s spokeswoman, Maria Zakharova, condemned the U.K. for setting a deadline for Russia to explain how the nerve agent could have been used to poison former spy Sergei Skripal, 66, and his daughter, Yulia, 33. 

Zakharova said on television Tuesday, “No one can come to parliament and say, ‘I give Russia 24 hours.’ ” Russia said it won’t respond to a U.K. ultimatum unless it gets samples of the nerve agent. 

The chemical is part of a group of military-grade nerve agents developed in the Soviet Union and known as "Novichok" — "newcomer" in English.

More: Novichok: What is Russia's dangerous poison used in ex-spy Sergei Skripal's case?

Earlier Tuesday, Russian Foreign Minister Sergey Lavrov insisted Moscow "is not to blame" and referred to the allegation as "nonsense."

His comments came before a midnight deadline set by May for the Kremlin to answer British claims that it was "highly likely" Russia was involved in the poisoning of the Skripals. Both remain in critical condition after the attack in Salisbury, England, on March 4.

May said Britain was ready to take "extensive measures" against Moscow if it did not get a satisfactory explanation for the attack. Britain would consider the incident "an unlawful use of force by the Russian state against the United Kingdom," she said.

British investigators provided few other details about why they suspect Russian involvement.

Outgoing Secretary of State Rex Tillerson said the United States agreed with Britain's assessment that Russia was probably behind the poisoning of the double agent. 

In a more cautious response, the White House said the case was an outrage, but it did not mention Russia. Trump said before departing for a trip to California, "It sounds to me like they (Britain) believe it was Russia."

Russian news agencies reported Tuesday that the nation's Foreign Ministry summoned Britain's ambassador in Moscow over the case, a tit-for-tat move that follows May's summoning of Russia's ambassador in London on Monday night. 

More: Secretary of State Rex Tillerson ousted in favor of CIA chief, Trump announces

Skripal was jailed in Russia in 2006 after he confessed to being recruited by British intelligence and supplying information about Russian agents. He was freed in 2010 as part of a U.S.-Russian spy swap and moved to Britain. 

It is not clear why Skripal was targeted, but the case is reminiscent of the poisoning death of another former Russian agent in 2006. Alexander Litvinenko was exposed to a rare radioactive isotope, polonium-210, at a London hotel. An official British inquiry concluded in 2016 that Russian President Vladimir Putin probably approved the assassination of Litvinenko.

A number of outspoken critics of Putin have been killed or died in mysterious circumstances, including journalists, opposition politicians and exiled tycoons.

Sen. Ben Sasse, R-Neb., a member of the Senate Armed Services Committee, said in a statement that "Putin’s shadow war is real. Sometimes it looks like information operations aimed at undermining confidence in self-government, sometimes it looks like ground operations with masked gunmen and unmarked uniforms in Eastern Europe, and sometimes it looks like an assassination attempt in Great Britain."

More: Russia faces midnight deadline to explain ex-spy's poisoning