The man accused of shooting three people to death and wounding nine others at a Planned Parenthood clinic in Colorado last month declared himself guilty and a "warrior for the babies" during an outburst in court on Wednesday.

The disturbance, as prosecutors formally presented murder and other charges against Robert Lewis Dear, 57, bolstered assertions by Planned Parenthood executives that the attack on the Colorado Springs clinic was motivated by anti-abortion sentiments.

Dear has been held without bond since surrendering to police at the end of a five-hour siege on Nov. 27 that authorities said began when he opened fire with a rifle in front of the clinic, then stormed inside.

It was the first deadly assault on a U.S. abortion provider in six years, since the 2009 assassination of a doctor at a Kansas church. Three people, including a police officer, were fatally shot and nine were wounded; no Planned Parenthood staff were killed or injured in the rampage.

Dear's outburst took place at the start of his hearing in El Paso County court on Wednesday. He was not scheduled to formally enter a plea during the day's proceedings.

"I'm guilty, there's no trial. I'm a warrior for the babies," Dear blurted out in a loud voice while seated beside his lawyers in front of the judge.

Addressing the judge, defence attorney Dan King raised the issue of Dear's mental competency to stand trial, saying, "I think the problem is obvious."

A search warrant accompanying police affidavits filed in the case has been placed under court seal, and authorities have yet to publicly disclose a motive for the shooting.

Colorado Springs Shooting

A man pauses after laying flowers in honour of the victims of a Colorado Springs Planned Parenthood shooting. A young mother, a veteran and a police officer were killed in the attack. (Daniel Owen/Associated Press)

Several media outlets, citing law enforcement sources, have reported that Dear uttered the phrase "no more baby parts" in statements to investigators following his arrest.

Dear, a native of South Carolina who once earned a living as a self-employed art salesman, appeared at Wednesday's hearing in shackles wearing turquoise-coloured jail garb.

At one point, during a discussion between the judge and attorneys about the sealing of documents in the case, Dear spoke out again.

"Seal the truth, huh? Kill the babies, that's what Planned Parenthood does," Dear said.