"President Trump pledged the full cooperation and support of the United States Government in responding to the attack and bringing those responsible to justice," the White House said in a readout of the call.
During the call, Trump praised the effective response of British security forces.
"We obviously condemn today's attack in Westminster, which the United Kingdom is treating as an act of terrorism and we applaud the quick response that the British police and their first responders made to the situation," White House press secretary Sean Spicer said at his daily press briefing.
"On behalf of the US, I express my condolences to the victims and their families. The American people send their thoughts and prayers to the people of the United Kingdom," he said.
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"We condemn these horrific acts of violence, and whether they were carried out by troubled individuals or by terrorists, the victims know no difference," Tillerson said in a statement.
The Department of Homeland Security said it stands in solidarity alongside its friends in the United Kingdom in condemning the terrible attack at Parliament.
At least three people were killed and 20 injured as terror struck the British capital when a man mowed down pedestrians on a bridge and stabbed a police officer outside parliament complex before being shot dead by Scotland Yard officers.