(CNN) -- Guantanamo Bay detainee Omar Khadr pleaded guilty to charges against him Monday, the Pentagon said, in the first military commission trial there since Barack Obama became president.

Khadr, 24, was accused of throwing a grenade during a 2002 firefight in Afghanistan that resulted in the death of Army Sgt. 1st Class Christopher Speer, a Special Forces medic.

He also admitted that he "converted landmines to Improvised Explosive Devices and assisted in the planting of 10 IEDs with the intent of killing American forces" in the months before killing Speer, the Pentagon said.

Khadr, the youngest detainee at Guantanamo Bay, was 15 at the time. He faced a maximum sentence of life in prison.

He pleaded guilty to murder in violation of the laws of war, attempted murder in violation of the laws of war, conspiracy, two counts of providing material support for terrorism and spying in the United States, a Canadian diplomat said.

Canada -- where Khadr was born -- has been closely involved in negotiations with the United States over his plea.

A source close to the case told CNN on Monday that the deal includes an eight-year prison sentence -- one year in U.S. custody and seven to be served in Canada.

Pentagon spokesman Col. Dave Lapan said he expects the sentencing phase of Khadr's trial to start Tuesday, adding that it probably will finish sometime this week.

Details of the plea agreement are not made public, Lapan said, because the seven military officers on the jury "get the case without any knowledge of the pretrial agreement. They will issue a sentence for the record, and after that -- if the judge allows it -- the pretrial agreement can be revealed."

If the jury's sentence is different from the plea agreement, the shorter sentence will be imposed, Lapan said.

"During the next several days, I look forward to proving to the panel and the world that Omar Khadr is a kind, compassionate and decent young man who deserves a first chance at a meaningful life," his military defense attorney, Army Lt. Col Jon Jackson, said in an e-mailed statement.

The military court in August viewed a 30-minute video that the government said shows Khadr helping to assemble and plant roadside bombs targeting American troops in Afghanistan. The video is undated.

The tape was discovered at the compound where Khadr was captured in 2002, a month after the firefight in which Khadr was seriously wounded and Speer was killed, the military said.

The video was allowed into evidence only after a flurry of defense objections, and the judge warned that it was difficult to tell who was speaking at different points on the tape.

One of Khadr's Canadian attorneys indicated last week that he was open to a plea deal.

"He is anxious to avoid a trial before that kangaroo court," Nate Whitling said in reference to the U.S. military commission at the Guantanamo Bay, Cuba, detention facility.

Catherine Loubier, a spokeswoman for the Canadian Foreign Ministry, offered no details about Khadr's plea, saying: "This matter is between Mr. Khadr and the U.S. government, and we will not have further comment on this today."

U.S. Secretary of State Hillary Clinton reached out to Canadian officials last week as part of an effort to resolve the case, according to two sources.

CNN's Jeanne Meserve, Chris Lawrence, Carol Cratty, Charley Keyes, Elise Labott and Jill Dougherty contributed to this report.