WASHINGTON, Jan. 6 (CNA) - A woman has died after being shot on Wednesday (Jan 6) during the storming of the US Capitol by supporters of President Donald Trump, a spokeswoman for the Washington police department said.

No other details were available, the spokeswoman said. It remained unclear who shot the woman, but a witness said the shooting occurred after law enforcement authorities urged people to "get back".

A protester from New Jersey told Washington CBS affiliate WUSA he witnessed the woman being shot at the Capitol after police and other law enforcement officials warned people to get back.

"Well, we had stormed into the chambers inside, and there was a young lady who rushed to the windows; a number of police and Secret Service were saying, 'Get back, get down, get out of the way,'" he said. "She didn't heed the call and as we kind of raced up to grab people and pull them back, they shot her in the neck, and she fell back on me."

The man told the local television station he saw blood coming from the woman's mouth and neck and nose.

“It could have been me, but she went first,” he said.

Hundreds of Trump supporters entered the Capitol on Wednesday in a bid to overturn his election defeat, forcing Congress to postpone a session that would have certified Democratic President-elect Joe Biden's victory in the Nov 3 election.

Video showed the Trump supporters breaking windows and police deploying tear gas inside the building.

Washington Metropolitan Police Chief Robert Contee said members of the crowd used chemical irritants to attack police and several had been injured. The FBI said it had disarmed two suspected explosive devices.

It was the most damaging attack on the iconic building since the British army burned it in 1814, according to the US Capitol Historical Society.

Police declared the Capitol building secure shortly after 5.30pm (6.30am Singapore time), more than three hours after it was breached.

House Speaker Nancy Pelosi said on Wednesday that lawmakers would resume the count of electoral votes to confirm the November election result once the US Capitol is cleared after pro-Trump rioters breached the building.

She said that decision was made after consultations with fellow congressional leaders and calls to the Pentagon, Justice Department and Vice President Mike Pence.

"We always knew this responsibility would take us into the night. The night may still be long but we are hopeful for a shorter agenda, but our purpose will be accomplished," she said in a statement.