The US Department of Homeland Security declared a nationwide terrorism alert Wednesday, citing the potential threat from domestic anti-government extremists opposed to Democrat Joe Biden as president.

Extremists "emboldened" by the deadly January 6 assault on Congress by angry supporters of former president Donald Trump could undertake attacks against elected officials and government facilities, the alert said.

The warning came as authorities in California charged a Trump supporter and follower of a far-right militia group with possession of five home-made pipe bombs, alleging he intended to attack Democrats.

The National Terrorism Advisory System Bulletin said the threat of attacks could persist for weeks, in the wake of Biden's January 20 inauguration and the storming of the US Capitol.

"Information suggests that some ideologically-motivated violent extremists with objections to the exercise of governmental authority and the presidential transition, as well as other perceived grievances fueled by false narratives, could continue to mobilize to incite or commit violence," the Department of Homeland Security said. 

Some security worries surround the impeachment trial of Trump beginning in the second week of February.

Trump has been charged by the House of Representatives with "incitement of insurrection" for allegedly encouraging the assault on the Capitol.

DHS said it had no information indicating any specific, credible threat. 

"However, violent riots have continued in recent days and we remain concerned that individuals frustrated with the exercise of governmental authority and the presidential transition... could continue to mobilize a broad range of ideologically-motivated actors to incite or commit violence," it said.

- Over 150 arrested -

The alert came two days after the Pentagon said thousands of National Guard troops deployed in Washington for Biden's inauguration would remain in the capital through March due to FBI intelligence on potential threats.

It was the first alert ever issued for domestic violent extremists.

Bennie Thompson, chairman of the House Homeland Security Committee, said the alert marked a turn from the previous four years of the Trump administration when the threat from the extreme right was downplayed, despite a growing number of deadly attacks.

"I am glad to see that DHS fully recognizes the threat posed by violent, right-wing extremists, and is taking efforts to communicate that threat to the American people," Thompson said.

The DHS alert said threats had grown since last year from domestic violent extremists motivated by Covid-19 restrictions, Biden's defeat of Trump in the November election, police brutality and illegal immigration.

On Tuesday, the Justice Department said it had arrested more than 150 people over the attack on the Capitol and was investigating hundreds more.

Increasingly the investigations are focused on conspiracy and sedition charges, which can bring up to 20 years in prison, said Michael Sherwin, the acting federal prosecutor for Washington.

Three people linked to the extremist Proud Boys and Oath Keepers groups have been charged on those grounds. Their case is significant, Sherwin said, because "it shows militia groups actively involved in planning and breaching the Capitol."

The FBI continues to search for a person or persons who placed two pipe bombs near the US Capitol on January 6. The bombs were real but never detonated, and the FBI has offered a $75,000 reward in the case.

In San Francisco Wednesday the Justice Department announced charges against Ian Benjamin Rogers, 43, for possessing five pipe bombs and the materials to make more.

An FBI affidavit said Rogers was linked to the Three Percenters violent anti-government group and had 49 guns and thousands of rounds of ammunition in his Napa Valley home and business.

Messages on Rogers' phone indicated he believed Trump's claim that Biden's November 3 election victory was rooted in widespread fraud, the affidavit said.

The messages showed "his intent to attack Democrats and places associated with Democrats in an effort to ensure Trump remained in office," it said.

The messages indicated Rogers was considering attacking the office of California Governor Gavin Newsom, a Democrat, and also of Twitter and Facebook, which blocked Trump's postings and those of right-wing extremists.

- Guilty plea in Michigan plot -

Meanwhile, one of six members of an armed militia group charged with plotting to kidnap Michigan Governor Gretchen Whitmer last year pleaded guilty Wednesday.

The plot against the Democratic governor was one of several cases which elevated concerns about far-right groups. 

In a plea deal, Ty Garbin admitted he was part of the conspiracy to kidnap and possibly even kill Whitmer due to her policies, including on Covid-19 restrictions.

Facing possible life in prison, Garbin agreed to cooperate in investigations of others in the case or related activities.