Washington, Apr 3 (IANS): A police officer was killed after a person rammed his vehicle into barriers surrounding the US Capitol complex, according to authorities.

The driver, identified as Noah Green, a 25-year-old man from Indiana, was shot by police at the scene on Friday afternoon and died in hospital, reports Xinhua news agency.

Green is a Nation of Islam follower, according to MSNBC, citing Facebook content that was thought to be posted by him.

The authorities have not yet specified a motive.

The slain police officer was identified by Capitol Police as 18-year veteran William "Billy" Evans.

"It is with profound sadness that I share the news of the passing of Officer William 'Billy' Evans this afternoon from injuries he sustained following an attack at the North Barricade by a lone assailant," acting Capitol Police Chief Yogananda Pittman said at a news conference.

"This has been an extremely difficult time for US Capitol Police," Pittman said, noting the attack underscores that the Capitol Hill remains a potential target for violence.

After ramming his car into the barricade, the suspect exited the vehicle with a knife, started lunging at and was killed by the police, he said.

"The suspect entered what we refer to as the north barricade of the Capitol. The suspect rammed his car into two of our officers and then hit the north barricade barrier.

"At such time the suspect exited the vehicle with a knife in hand. Our officers then engaged that suspect. He did not respond to verbal commands," Pittman added.

The attack occurred about 91 metres from the entrance of the building on the Senate side of the Capitol.

Immediately after the attack the complex was placed under a lockdown. It was lifted two hours later.

White House Press Secretary Jen Psaki told reporters that President Joe Biden, who landed at Camp David on Friday afternoon to spend the Easter weekend, is "aware" of the Capitol incident.

House Speaker Nancy Pelosi ordered the flags at the Capitol to be flown at half-staff to mourn the death of the police officer.

Following the attack, there was a heavy police response outside of the Capitol, including a helicopter landing on the premises.

Washington Field Office of the Federal Bureau of Investigation said it also provided support to the Capitol Police.

The D.C. National Guard also deployed an Immediate Response Force to the complex to support the Capitol Police.

It was the second line-of-duty death in the Capitol Police this year.

Capitol Police officer, Brian D. Sicknick, died a day after the January 6 riot staged by supporters of former President Donald Trump.

He passed away "due to injuries sustained while on-duty".

Two other Capitol Police officers have died by suicide in the wake of the riot.

Barbed wire fencing has been set up around the Capitol complex since the riot.

Several thousands of the National Guard troops still remain in Washington, D.C. over security concerns.