Story highlights Trump has been briefed on the incident by his national security adviser

House Speaker Paul Ryan offered his condolences to the victims

Washington (CNN) US officials are offering assistance to British authorities after an attack Wednesday on the Houses of Parliament in London that police there are treating as terrorism.

President Donald Trump has been briefed on the incident by his national security adviser H.R. McMaster, according to a National Security Council official, and US officials are in touch with their counterparts in the UK.

At least three people, including a police officer, were killed, and at least 20 injured, Acting Deputy Commissioner of the Met Police Mark Rowley told reporters outside Scotland Yard.

He added the police currently believe there was only one attacker, who was killed, but are continuing to investigate.

President Trump spoke to British Prime Minister Theresa May following the attack, according to White House Press Secretary Sean Spicer.

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