Former acting defense secretary Christopher Miller plans to say the military was deliberately restrained on the day of Trump's January 6 rally, which turned into an assault by hundreds of his followers that left five dead including a police officer.

"I am keenly aware of the criticism regarding the Department of Defense's response," according to a copy of his remarks.

"My concerns regarding the appropriate and limited use of the military in domestic matters were heightened by commentary in the media about the possibility of a military coup or that advisers to the president were advocating the declaration of martial law."

Miller will testify on Wednesday at a hearing held by the House of Representatives oversight committee on unanswered questions from the attack.

National Guard troops did not arrive at the Capitol until about 5.20pm on January 6, more than four hours after the grounds were first breached by the pro-Trump mob.

Lawmakers have held a series of hearings on the federal government's preparations for and response to the attack, leading to finger-pointing between city officials, federal law enforcement agencies, and military leaders.

Major General William Walker, the District of Columbia's National Guard commander, testified before a Senate panel in March that Pentagon officials took more than three hours to approve a request for National Guard troops to back up police.

Miller is expected to testify that the Pentagon's response was "rapid" compared with other military deployments in urban areas.

He is expected to fault Trump for encouraging the violent attack.

"I stand by my prior observation that I personally believe his comments encouraged the protesters that day."

Australian Associated Press