WASHINGTON－A US federal grand jury charged four leaders of the far-right Proud Boys with conspiring to block Congress from certifying President Joe Biden's election on the day of a deadly assault on the Capitol, court papers unsealed on Friday said.

The indictment alleges that Ethan Nordean of Washington, Joseph Biggs of Florida, Zachary Rehl of Pennsylvania and Charles Donohoe of North Carolina conspired to encourage members of the group to attend the Stop the Steal protest in Washington on Jan 6.

All four defendants are leaders or organizers of Proud Boys chapters in their respective states, the indictment released on Friday said.

It says they worked to obtain paramilitary equipment used for the attack on the US Capitol, dismantled metal barriers set up to protect the building, and communicated using handheld radios and encrypted messaging applications.

It also says the effort included soliciting donations through an online crowdfunding campaign to help the Proud Boys pay for protective gear, and an online fundraiser that generated more than $5,500 to help cover travel expenses to Washington.

More than 300 people have been charged over the attack, which left five people dead after a mob of supporters of former US president Donald Trump stormed the building in a failed attempt to stop Congress from certifying Biden's victory.

Approximately 20 people charged to date are associated with the Proud Boys, and some of the others were tied to anti-government militias such as the Oath Keepers and the Three Percenters.

The indictment alleges they made their intentions clear as far back as November, with Biggs declaring in a social media post on Nov 5, threatening war if the election was stolen from Trump.

On Nov 27 Nordean declared: "We tried playing nice and by the rules, now you will deal with the monster you created."

On Jan 5 a new encrypted messaging channel called Boots on the Ground was created, and more than 60 users participated, including the four defendants and a fifth unindicted co-conspirator, the indictment says.

It says that later that day the unnamed co-conspirator sent a message telling everyone that Nordean, who also goes by the name Rufio Panman, would be a leader in the effort.

The next day, the indictment alleges, they advanced toward the Capitol, knocked down metal barricades and eventually made their way into the Capitol.

Nordean and Biggs had previously been arrested on criminal complaints.

Earlier this month, however, the Justice Department lost its bid to keep Nordean detained pending trial, after a federal judge said the government had failed to substantiate allegations that Nordean was a ringleader of the attack.

Biggs was arrested in January and released on a $25,000 bond.

Agencies via Xinhua