Donald Trump has said the US is “with the UK all the way” over the poisoning of Sergei Skripal and his daughter, Downing Street said.
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In a phone call with Theresa May, Mr Trump agreed the Russian Government “must provide unambiguous answers as to how this nerve agent came to be used”.
Mr Trump made the comments when he spoke to Mrs May by telephone on Tuesday afternoon.
A Downing Street spokesman said: “The Prime Minister spoke to President Trump earlier this afternoon to update him on the ongoing investigation into the Salisbury incident.
“The Prime Minister set out the conclusion reached by the UK Government that it was highly likely that Russia was responsible for the attack against Sergei and Yulia Skripal.
“President Trump said the US was with the UK all the way, agreeing that the Russian government must provide unambiguous answers as to how this nerve agent came to be used.”
Earlier in the day, Trump had said: “It sounds to me like it would be Russia based on all of the evidence they have.”
He was questioned after he sacked Secretary of State Rex Tillerson just hours after Mr Tillerson strongly supported the UK.
The President told reporters outside the White House: “Theresa May is going to be speaking to me today.
“It sounds to me like they believe it was Russia and I would certainly take that finding as fact.”
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The President, who had initially avoided blaming the Kremlin over the assassination attempt against former double agent Sergei Skripal, had announced on Twitter that Mr Tillerson would be replaced by CIA director Mike Pompeo.
The Washington Post reported that the decision was made on Friday and Mr Trump said he and Mr Tillerson, who had reportedly once called the President a ‘moron’, had been “talking about this for a long time” and had disagreed on issues like the Iran deal.
But with the Trump camp’s links with Moscow facing continued scrutiny, the timing of the announcement prompted questions about the President’s support for the UK in the row with Russia.
The former chair of the House of Commons Foreign Affairs Committee Mike Gapes said it appeared that Mr Tillerson may have suspected he was about to be sacked at the time he made his statement backing the UK.
“It looks as though he used one of his last acts in post to do the right thing and say the right thing about the attempted murders in Salisbury,” Mr Gapes told the Press Association.
The news came as the international community rallied round Britain after the first use of this kind of weapons grade nerve agent in Europe since the Second World War.
A huge police inquiry was launched after Mr Skripal, 66, and his daughter Yulia, 33, were found slumped on a bench in Salisbury, Wiltshire, on March 4. They remain in a critical condition.
Detective Sergeant Nick Bailey, who was part of the initial police response, is in a serious but stable condition.
The UK’s head of counter-terrorism policing Neil Basu said the investigation will take “many weeks”.
Speaking at Scotland Yard, he explained that the investigators’ “prime focus” is how the poison was administered.
He said Miss Skripal had flown to the UK the day before the attack and police are also are focusing on Mr Skripal’s red BMW and appealing for any witnesses who saw the pair in the car between 1pm and 1.45pm on the day of the poisoning to come forward.
The episode has left Britain’s relations with Moscow, which were already under severe strain, at breaking point.
On Monday, Prime Minister Theresa May set a deadline of midnight on Tuesday for the Russians to provide a credible explanation as to why deadly Novichok was used on UK soil.
Peter Wilson, the UK Permanent Representative to the Organisation for the Prohibition of Chemical Weapons, told fellow delegates: “I did not expect to have to brief this Council on the first offensive use of a nerve agent of any sort on European territory since World War Two.
“The stark conclusion is that it is highly likely that Russia, a fellow State Party to the Chemical Weapons Convention (CWC), is implicated in chemical weapons use, whether by failure to control its own materials or by design.
“This attempted murder, using a weapons-grade nerve agent in a British city, was not just a crime against the Skripals. It was an indiscriminate and reckless act against the UK, which put the lives of innocent civilians at risk.”
Foreign Secretary Boris Johnson said if Russia failed to provide “a convincing explanation” the UK response would be announced on Wednesday.
He added: “Russia is a great country. It is a great pity therefore that the Russian regime seems to be moving in this dangerous and disruptive direction.”
The Government has a variety of counter-measures including economic, financial, and diplomatic moves. There has also been speculation about a potential covert cyber offensive against Russia in retaliation.
Mrs May told the regular weekly meeting of Cabinet at 10 Downing Street that there was “no doubt of the severity of what had taken place in Salisbury, which was a reckless, indiscriminate and despicable act”.
The PM confirmed that she will chair a meeting of the National Security Council on Wednesday to discuss the Russian response and will then inform the House of Commons of any measures to be taken.
France, Germany and Mr Tillerson all gave their backing to the UK after the Government confirmed it was “highly likely” Russia was behind the assassination attempt.
However Russian foreign minister Sergei Lavrov insisted the country was not to blame, and asked for access to samples of the poison.
News agency Tass quoted Russian foreign ministry spokeswoman Maria Zakharova as saying: “It is a circus show in the British Parliament.
“The conclusion is obvious, it’s another political information campaign, based on a provocation.”