Donald Trump has said Theresa May is "strong and doing very well" after the two leaders spoke by phone following the terror attacks in Westminster.

The US President tweeted late on Wednesday night: "Spoke to U.K. Prime Minister Theresa May today to offer condolences on the terrorist attack in London. She is strong and doing very well."

Earlier, White House spokesman Sean Spicer applauded "the quick response of British police and first responders”.

Mr Trump himself said during a brief appearance before reporters at the White House that he was just getting the news, calling it "big news”.

French President Francois Hollande offered his condolences to Ms May for those who died in the attacks and expressed France's solidarity with Britain "in this tragic ordeal".

"The British and French services are in close contact to conduct the investigation," he added.

German Chancellor Angela Merkel also expressed her support and solidarity with Britain.

"I want to say for Germany and its citizens: we stand firmly and resolutely by Great Britain's side in the fight against all forms of terrorism,” she said.

Canadian Prime Minister Justin Trudeau said: "Our thoughts are with the victims of today's attack in London and their families.