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.