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.

In addition to offering his condolences, Trump pledged the full cooperation and support of the US government in “responding to the attack and bringing those responsible to justice” during the call.

In addition to pledging full US support in bringing those responsible “to justice,” Spicer also condemned the violent acts and applauded the the quick response of British police and first responders during his daily press briefing on Wednesday.

The US State Department echoed the White House in denouncing the attack.

“We condemn these horrific acts of violence, and whether they were carried out by troubled individuals or by terrorists, the victims know no difference,” US Secretary of State Rex Tillerson said in a statement.

But several US lawmakers said that they are waiting to find out more about who was behind the attack.

“Was it a mentally disturbed person? Was it a domestic case? They say it’s a terrorism case – motives matter,” Republican Sen. Lindsey Graham of South Carolina told CNN.

“As the details emerge, we’ll have to adjust what we do.” said Democratic Sen. Jack Reed of Rhode Island.

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Trump told the White House press pool that he was getting an update on the situation ahead of attending a roundtable on women’s health.

“Some big news having to do with London just happened,” said Trump.

A senior Trump administration official told CNN that the US assistance would likely come from assets that are permanently stationed in London, should the UK accept the offer of help, but didn’t rule out people being dispatched from the United States.

The US State Department is also closely monitoring the situation and “stands ready to assist in any way the UK authorities would find helpful,” acting spokesman Mark Toner said.

“The safety and security of US citizens overseas is one of our highest priorities,” he told CNN. “We urge US citizens in London to contact family members and loved ones to notify them that you are safe, to avoid the area and monitor local news.”

Domestically, US security posture has not changed, according to a statement from the Department of Homeland Security. However, US law enforcement is also in close contact with UK authorities to monitor the events and “support the ongoing investigation.”

House Speaker Paul Ryan offered his sympathies to the victims of the attack via Twitter.

“Terrible scene in London. Praying for the victims of this apparent act of terror,” Ryan wrote. “We stand with our friends in Parliament and Great Britain.”

British Foreign Secretary Boris Johnson was notably absent from a photo-op following Wednesday’s meeting with the Global Coalition on ISIS taking place at the State Department in Washington. It is unclear whether Johnson left the event early due to the situation in London.

CNN’s Jeremy Herb, Michelle Kosinski and Dan Merica contributed to this report.