The US Capitol building is in lockdown after a vehicle reportedly slammed into the security barriers surrounding the complex.

A police officer has been killed and another has been injured after a car rammed into the US Capitol complex.

The vehicle crashed into a security fence before the driver lunged towards the officers with a knife, police said.

The officers then opened fire and the suspect, who has not been identified, was shot dead.

Police have said that a suspect is in custody after reports that someone "rammed a vehicle" into two officers who have since been taken to hospital.

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In a press conference on Friday, the acting chief of police in Washington DC said the attack did not appear to be related to terrorism.

"It is with a very, very heavy heart that I announce one of our officers has succumbed to his injuries," Acting Capitol Police Chief Yogananda Pittman said at the conference.

The exact circumstances of the officer's death are not yet clear.

"Whether the attack was at law enforcement, or whoever, we have a responsibility to get to the bottom of it and we'll do that," Robert Contee, the acting chief of the city's Metropolitan Police Department, said.

There is an increased security presence around the Capitol building, where the US Congress sits, and a number of police cars surrounding the area.

The Capitol and surrounding streets in Washington DC are in lockdown after police warned of an "external security threat" as onlookers were told to clear the area.

The reports come nearly three months after the fatal 6 January riots at the Capitol.