US Capitol Police chief sorry for riot

The acting head of US Capitol security has apologised for "failings" during the deadly attack on the building by followers of then-president Donald Trump in a bid to stop the certification of Joe Biden's election victory.

She specifically acknowledged a number of missteps: conflicting intelligence, inadequate preparation and insufficient mobilisation of partner agencies, and called for improving accountability systems and communications structures.

"I am here to offer my sincerest apologies on behalf of the department," said Yolanda Pittman, the acting chief of Capitol Police, according to a prepared statement for the US House of Representatives' appropriations committee.

"The department failed to meet its own high standards as well as yours."

Pittman said many of the officers are suffering from post-traumatic stress disorder after the January 6 assault in which five people died, including one Capitol police officer. Pittman said the death of a second officer was indirectly linked.

She and Timothy Blodgett, the acting US House of Representatives' serjeant-at-arms, said security officials were working to do more to boost protection of the US Capitol, the seat of government.

Pro-Trump supporters stormed the building following Trump's urgings at a rally near the White House to go to the Capitol. Trump was impeached by the House on a charge of incitement and the trial in the Senate is scheduled for February.

The then-chiefs of the Capitol Police and House sergeant at arms stepped down.

In the weeks since the attack, security has been heightened around the Capitol and in Washington in general, with high fencing surrounding the perimeter and National Guard troops brought in for Biden's inauguration on January 20.

About 5000 National Guard troops will remain in Washington through mid-March.