Donald Trump has pledged “solidarity” with Britain over the Salisbury spy poisoning and demanded “unambiguous answers” from Russia over its involvement.
The US president offered the UK “any assistance” needed in its investigation in a phone call with Theresa May, according to the White House.
Both leaders said there should be “consequences” for whoever attacked Sergei Skripal and his daughter Yulia on British soil.
The pair also agreed it amounted to a “flagrant violation of international norms” – a hint of more serious diplomatic action to come.
A White House source told The Daily Telegraph the offer of investigative help was the first in a series of US moves expected in the coming weeks.
The call came as America’s stance was plunged into doubt when Rex Tillerson was sacked as secretary of state after publicly blaming Russia.
Mr Tillerson had said that Russia was “clearly” linked to the attack on Monday, going further than the White House. On Tuesday morning it was announced he had been fired by the president.
Boris Johnson talked to Mr Tillerson for 20 minutes about the poisoning on Monday evening and was not told of his impending departure, adding to the confusion.
Amid shifting US statements, Mr Trump further muddied the waters on Tuesday by initially saying he would “condemn Russia or whoever it may be” once the facts become clear.
However after a call with Mrs May, the White House issued its strongest statement yet on the poisoning, including a direct challenge to Russia.
“President Trump stated the United States stands in solidarity with its closest ally and is ready to provide any assistance the United Kingdom requests for its investigation,” a White House spokesman said of the call.
“President Trump agreed with Prime Minister May that the Government of the Russian Federation must provide unambiguous answers regarding how this chemical weapon, developed in Russia, came to be used in the United Kingdom.
“The two leaders agreed on the need for consequences for those who use these heinous weapons in flagrant violation of international norms.”
A Downing Street spokesman said: “President Trump said the US was with the UK all the way.”
Angela Merkel, the German chancellor, also told Mrs May “she stood in full solidarity” with Britain in a call on Monday, according to Number 10.
Mr Tillerson, a former oil executive, will be replaced by Mike Pompeo, the director of the CIA, providing his appointment is confirmed by the US Senate.
There has been speculation for months that Mr Trump wanted Mr Tillerson gone after clashing publicly over North Korea and other policies.
Mr Tillerson also repeatedly refused to deny he called Mr Trump a “moron” last year.
Mr Tillerson’s sacking triggered a war of words between the State Department and the White House over how he was informed.
Steve Goldstein, the under secretary of state, released a statement saying Mr Tillerson “did not speak” to the president before being fired and was “unaware” of the reasons for his dismissal.
There were even reports that Mr Tillerson only learned of his sacking when Mr Trump tweeted it on Tuesday morning.
However White House officials insisted John Kelly, Mr Trump’s chief of staff, suggested to Mr Tillerson on Friday and Saturday that the president would act soon.
Mr Goldstein was fired on Tuesday over his statement. It later emerged Mr Tillerson received a phone call from Mr Trump more than three hours after his sacking was announced.
Appearing to address his departure on Tuesday, Mr Tillerson said he would formally leave his post on March 31 and criticised Russia for its “troubling” actions.
“Russia must asses carefully how its actions are in the best interests of its people and the world more broadly,” Mr Tillerson said.
“If it continues on this current trajectory it is likely to lead to greater isolation which is not in anyone’s interests.”