Donald Trump has vowed to "condemn Russia or whoever" was behind the Salisbury nerve agent attack, as Boris Johnson refused to rule out UK cyber attacks against Moscow.

The US President said the poisoning on British soil "sounds" like the work of Russia, adding that he would talk to Prime Minister Theresa May on Tuesday.

He said he will condemn "Russia or whoever it may be" once "we get the facts straight" about who was behind the attack with a weapons-grade nerve agent that left Sergei Skripal and his daughter Yulia in hospital.

President Trump, who initially avoided blaming the Kremlin over the assassination attempt in Salisbury, announced on Twitter on Tuesday that Rex Tillerson, the US Secretary of State, has been sacked after publicly backing the UK finding that Russia is "highly likely" to have been behind the attack.

Earlier, Boris Johnson, the Foreign Secretary, warned Moscow not to underestimate Britain’s outrage over the use of a nerve agent and welcomed international support after France and Germany also gave their backing to the UK.

Video: Boris Johnson on Russia's midnight deadline