Donald Trump talks with British PM on London attack

The White House said on Wednesday US President Donald Trump spoke with British Prime Minister Theresa May after a deadly attack in London killed at least four.

Speaking at the daily briefing, White House spokesman Sean Spicer said the UN condemned the attack and applauded the quick response of British police and first responders.

In a statement released by the US State Department, US Secretary of State Rex Tillerson also condemned the "horrific acts of violence".

Meanwhile, US Department of Homeland Security said in a statement that the US would offer assistance to the British investigation, adding that the security posture in the country remained unchanged.

Earlier on Wednesday, a knifeman stabbed a police officer outside the house of parliament in London before police shot him dead. A vehicle crushed into people on the nearby Westminster Bridge, according to media reports.

At least four people were killed and 20 others injured in the "terrorist" attack, according to London police.