Thousands of US National Guard troops mobilized in Washington for the presidential inauguration will remain in place through mid-March due to ongoing threats, Pentagon officials said Monday.

The officials did not offer specific information on the threats, saying the information came from the FBI.

But Washington remained concerned about more possible violence after the January 6 attack on Congress by supporters of president Donald Trump, and ahead of the impeachment trial of Trump beginning the week of February 8.

Acting Army Secretary John Whitley said they had been briefed on possible risks with "several" upcoming events in Washington over the coming several weeks.

Security officials are concerned that protests "could either be used by malicious actors or other problems that could emerge," he said.

"We are posturing our forces to be able to respond to those threats if they emerge," he said.

The attack on the US Capitol, which left five people dead and has been labelled an insurrection, led the army to increase the number of National Guard troops deployed in Washington from a few hundred to 25,000 for the January 20 inauguration of President Joe Biden.

Much of the center of the city was locked down and the normal hundreds of thousands of attendees told to stay home due to the high level of security.

On Monday about 13,000 of the reservists remained.

About 7,000 would stay through the end of January, and then fall slowly to about 5,000 in mid-March, according to Whitley.

Asked about the specific threats, Whitley referred reporters to the FBI, which did not respond to requests for comment.

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