The judge in the trial of a man accused of murdering an abortion doctor dealt the defence a major setback Thursday, ruling that the jury cannot consider a lesser charge of manslaughter.

The ruling in a Wichita, Kan., courtroom came hours after Scott Roeder took the stand in his own defence and admitted killing Dr. George Tiller on May 31, 2009, in the foyer of a Wichita church the doctor attended. He had previously confessed publicly to the shooting.

Roeder said he acted to save the lives of unborn children.

Roeder's lawyers had hoped to win a lesser conviction of voluntary manslaughter, which requires them to show their client had an unreasonable but honest belief that deadly force was justified. The charge carries a considerably lighter sentence than murder.

Roeder has pleaded not guilty to first-degree murder and aggravated assault. Asked by his own lawyer about his views on abortion, the 51-year-old Roeder, who is from Kansas City, Mo., said he believes it amounts to murder.

"From conception forward, it is murder," Roeder told the court. "It is not a man's job to take life. It is our heavenly father's. He is our creator."

Roeder replied "yes" or "no" to many of the questions he was asked, and his attempts to elaborate drew frequent objections from prosecutors, who say Roeder lacks the medical expertise to describe Tiller's practice.

When asked to detail the types of abortion procedures he was familiar with, Roeder answered "four or five" and then listed them. In one instance, he described a procedure as involving the fetus being "torn limb from limb" — a characterization that prompted a quick objection from the prosecution.

Before Roeder took the stand, District Judge Warren Wilbert barred former Kansas attorney general Phill Kline, who is known for campaigning against abortion, from testifying after listening to a preview of Kline's testimony without the jury present.

Kline investigated Tiller's clinic, Women's Health Care Services, in 2006 because he suspected Tiller was violating Kansas's laws pertaining to late-term abortion. The case was later dropped because of jurisdictional issues.

Wilbert said allowing Kline's testimony at the trial would be "inappropriate" and said much of it amounted to "exactly what this court seeks to avoid."

"I said I would not allow this courtroom to turn into a forum or a referendum on abortion," Wilbert said.

The decision to bar Kline's testimony hampers the strategy of the defence, which had hoped to show that Roeder based his actions in part on Kline's belief that Tiller was breaking the law — a potential step toward a lesser conviction of voluntary manslaughter.

Wilbert had reminded Roeder's lawyers in court on Wednesday that they had to couple a voluntary manslaughter defence with a showing of imminent danger posed by the doctor. He noted abortion is legal in Kansas.

The two counts of aggravated assault Roeder faces are for allegedly using a gun to threaten two ushers who tried to stop him after the May 31 shooting.