U.S. Secretary of Defense Leon Panetta shakes hands with Vietnamese Foreign Minister Pham Binh Minh during a meeting at the Foreign Ministry in Hanoi, Vietnam. (Credit: AP)

In an historic agreement between two formerly warring nations, the Vietnamese government says it will open three new sites for excavation by the U.S. to search for troop remains from the war.

The announcement from Vietnam's defense minister comes as U.S. Secretary of Defense Leon Panetta and his Vietnamese counterpart exchanged artifacts collected during the war. They include letters written by an American soldier who was killed that had been kept and used as propaganda. There's also a small red diary kept by a Vietnamese soldier. A U.S. service member took the journal home. The two defense leaders agreed to return the papers to the families of the deceased soldiers.

U.S. officials say this is the first time such a joint exchange of war artifacts has occurred, and may bring some answers about our troops during those years.

Ron Ward, U.S. casualty resolution specialist at the Joint POW/MIA Accounting Command in Hanoi, said there are at least four U.S. troops believed to be lost in the three areas that were opened by the Vietnamese Monday. With those three areas now open, Ward said there are now just eight sites left that are still restricted by the Vietnamese.

Military officers briefing Panetta at the command's office said they had five to seven years to complete their excavation work. The acidic soil in Vietnam erodes bones quickly, leaving in many cases only teeth for the military teams to use to try and identify service members, one of the team members said.

In addition, many of the potential witnesses with information about remains are getting older and their memories are fading.

There are about nearly 1,300 cases that are still unaccounted for, and officers briefing Panetta said about 600 of those remains could be recoverable.