Donald Trump has offered the "full co-operation and support" of the US in responding to the Westminster terror attack and "bringing those responsible to justice".
The White House said the President offered his condolences to Theresa May in a phone call following the bloody events in London, which left four people dead including a policeman and knifeman.
Mr Trump had earlier described the apparent terror attack as "big news" as world leaders were shocked by the incident.
A White House spokesman said: "President Donald Trump spoke today with Prime Minister Theresa May of the United Kingdom to offer his condolences on today's terror attack in London and his praise for the effective response of security forces and first responders.
"He pledged the full co-operation and support of the United States government in responding to the attack and bringing those responsible to justice."
According to US network NBC, President Trump told reporters shortly after the attack: "I was just getting an update on London.
"Some big news having to do with London just happened."
Canadian Prime Minister Justin Trudeau said: "Our thoughts are with the victims of today's attack in London and their families.
"Canadians remain united with the people of the UK."
Gibraltar's Chief Minister Fabian Picardo said: "Shocked at attack on London. Yesterday I spent the day at Westminster. Luckily not today.
"London's Parliament is the mother of democracy!"
Earlier, Donald Trump Jr used the attack to criticise Sadiq Khan over old comments he made about terrorism.
