Confessions that Omar Khadr made under interrogation are admissible as evidence in the Toronto native's war-crimes trial in Guantanamo Bay, a military judge has ruled.

Khadr's lawyers had asked that the confessions be excluded from the proceedings. But judge Colonel Patrick Parrish ruled that the jury will be allowed to consider the confessions, as well as a video of Khadr allegedly making explosive devices.

That news came hours after Khadr officially pleaded not guilty Monday. His lawyer said the threat of rape and death by an interrogator should make his client's incriminating statements unreliable and inadmissible in court.

The accused war criminal entered the plea on all charges through his lawyer, Lt.-Col. Jon Jackson, as pretrial motions for the long-awaited trial came to a close in the Guantanamo courtroom.

If convicted, the 23-year-old could face life in prison.

Jackson had argued that Khadr's confessions should be ruled inadmissible because the interrogator has testified to telling Khadr that a young detainee in American custody was raped by other inmates, possibly to death.

The interrogation, involving former U.S. Army sergeant Joshua Claus occurred at Bagram prison in Afghanistan soon after American forces captured Khadr, the youngest detainee and the only Canadian in Guantanamo Bay.

"He was 15 years old when Interrogator 1 (Claus) told him that story," Jackson said. "Once he said those words ... the well is poisoned. The government can't cleanse the well."

Claus has since been convicted of detainee abuse.

Khadr was severely wounded when he was taken into custody by American troops at age 15, following a 2002 firefight in Afghanistan. He is accused of throwing a grenade that killed a U.S. soldier, and faces several charges of war crimes.

The prosecution said Khadr's assertions of abuse were lies, and said Khadr refused to take the stand to be cross-examined on the matter.

The confessions were freely given and reliable, Capt. Chris Eason told presiding judge Col. Patrick Parrish.

"He talked about the grenade. He talked about how he threw the grenade," Eason said.

"It's truth that we can rely on."

He also said Khadr's FBI interrogators never resorted to threats or even yelling to get a confession.

"The people here went to great lengths to act professionally," he said. "There was an atmosphere of rapport."

Opening statements will be made following jury selection, which is set to begin Tuesday.

Another motion asks that Khadr be given three-for-one sentencing credit because of alleged "illegal punishment" by the U.S. government.

Khadr appeared in court Monday, after both his military and Canadian lawyer earlier said they were not sure whether he would show up.

"He walked in surrounded by three guards and he seemed relaxed," CTV's Daniele Hamamdjian reported from Cuba. "He smiled at his military lawyer . . . and for the rest of the morning, he was reading a magazine on World Cup soccer."

Jackson has called the trial "the first case against a child soldier in modern history."

The trial is expected to last weeks.

Khadr has previously called the trial "a sham" and fired his American lawyers.

The trial is the first of its kind for U.S. President Barack Obama's administration. It comes 18 months after his promise to close the controversial facility.

On Friday, the U.S. Supreme Court refused to stay Khadr's trial, as the defence argued unsuccessfully that the U.S. Military Commission Act has not been legally tested by the nation's courts.

In Canada, the Conservative government has faced criticism for refusing to repatriate Khadr, the last Western detainee at Guantanamo Bay.