US president Donald Trump described the terror attack at Westminster as "big news" as world leaders reacted with shock to the incident.
Mr Trump was speaking after having an update on the situation.
The US State Department said it was ready to assist the UK authorities in any way.
According to US network NBC, Mr Trump told reporters: "I was just getting an update on London.
.@POTUS has been briefed on the situation in the U.K. Continuing to monitor and update— Sean Spicer (@PressSec) March 22, 2017
"Some big news having to do with London just happened."
State Department spokesman Mark Toner said: "We are following the concerning situation outside the UK Parliament.
"Our hearts go out to those affected. We stand ready to assist in any way the UK authorities would find helpful.
"The safety and security of US citizens overseas is one of our priorities. Our embassy in London is monitoring the situation closely."
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!"
Dutch Prime Minister Mark Rutte said: "Horrible images from London. The very heart of the city has been struck. "Our thoughts are with the British people."
Several EU leaders sent messages of concern and sympathy on social media.
Belgian Prime Minister Charles Michel, who earlier in the day had taken part in commemorations to mark the first anniversary of a terror attack in Brussels, said: "Our condolences are with those who mourn and all who are affected in London. Belgium stands with UK in fight against terror."
Danish PM Lars Lokke Rasmussen said: "Horrified by the images from London. My thoughts are with our British friends."
Lithuanian President Dalia Grybauskaite said: "Terrorism - weapon of cowards. Strongly condemn gruesome terrorist attack at the heart of Westminster."
The European Commission's chief Brexit negotiator Michel Barnier said: "My thoughts are with the people of London."
The US Department of Homeland Security said it "stands in solidarity alongside our friends in the United Kingdom in condemning the terrible attack at Parliament today".
It said: "With our partners in federal law enforcement, we are in close contact with our British counterparts to monitor the tragic events and to support the ongoing investigation."