Pool / Getty Images

U.S. Secretary of Defense Leon Panetta and Vietnam Minister of Defense Phung Quang Thanh at an arrival ceremony in Hanoi on Monday.

HANOI, Vietnam -- The search for U.S. servicemen missing from the Vietnam War was given a boost Monday when the Vietnamese government told visiting U.S. Defense Secretary Leon Panetta it would open three previously-closed sites to permit excavation for remains.

The announcement came as Panetta and Vietnam Defense Minister Phung Quang Thanh exchanged long-held artifacts collected during the war -- including letters written by a U.S. soldier who was killed that had been kept and used as propaganda, and a small maroon diary belonging to a Vietnamese soldier. A U.S. service member took the journal back to the U.S. 

Military officers briefing Panetta at the command's office said they had five to seven years to complete their excavation work in the previously restricted areas. 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. 

'I will ever forget the bloody fight': GI's letters provide a glimpse at fog of war

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. 

Ward said that opening the three new sites will enable the U.S. to try and find: 

• Two Air Force members who were lost when their plane was shot down in Quang Binh Province in central Vietnam in 1967. 
• An Army private first class who went missing when he was out with his unit on a search-and-destroy mission in 1968 in the tri-border area of Laos, Cambodia and Vietnam. 
• A Marine who was on a surface-to-air combat mission and was lost when his plane went down in Quang Tri Province. Another Marine on the plane ejected and was rescued. 

Panetta visits Vietnam, exchanges soldiers effects

During the press briefing announcing the expansion, both said their countries want to work together, whether or not the expanded relationship bothers China. 

Beijing has expressed concern over America's new defense strategy that puts more focus on the Asia-Pacific region, including plans to increase the number of troops, ships and other military assets in the region. 

The United States is looking to expand military ties with Vietnam after they signed last year a memorandum of understanding on defense cooperation.

On Sunday, Panetta became the most senior U.S. official since the end of the Vietnam War to visit Cam Ranh Bay in central Vietnam, a U.S. logistics hub during the conflict. He visited a U.S. Navy cargo ship that was undergoing repairs at the Vietnamese port.

Speaking through an interpreter, Thanh said Vietnam wants to continue defense cooperation with all countries, including stable and longstanding relationships with China and the United States. Hanoi, he said, would not sacrifice relations with one country for another. 

Panetta said the U.S. goal is to help strengthen the capabilities of countries across the region. 

Panetta: Majority of US warships moving to Asia

"Frankly the most destabilizing situation would be if we had a group of weak nations and only the United States and China were major powers in this region," said Panetta. 

Document exchange
Also on Monday, defense officials reviewing the packet of documents given to Panetta said it appears there are three sets of letters, including a set from the soldier, U.S. Army Sgt. Steve Flaherty, who was from Columbia, S.C. It was not clear how many other service members' letters were there, but officials were going through them Monday. 

Pool / Reuters

The letters of U.S. Army Sgt. Steve Flaherty, who was killed during the Vietnam war in 1969, are seen on a table at the Joint POW/MIA Accounting Command in Hanoi on Monday.

Officials said this is the first time such a joint exchange of war artifacts has occurred. The two defense leaders agreed to return the papers to the families of the deceased soldiers. 

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. 

Flaherty, who was with the 101st Airborne, was killed in the northern section of South Vietnam in March 1969. According to defense officials, Vietnamese forces took his letters and used them in broadcasts during the war. 

Vietnam's 'napalm girl' comes to terms with iconic photo

Vietnamese Col. Nguyen Phu Dat kept the letters, but it was not until last August, when he mentioned them in an online publication, that they started to come to light. 

Early this year, Robert Destatte, a retired Defense Department employee who had worked for the POW/MIA office, noticed the online publication, and the Pentagon began to work to get the letters back to Flaherty's family. 

Pool / Reuters

A picture sits next to a diary that belonged to Vietnamese soldier Vu Dinh Doan, which was originally taken from Doan's body by U.S. Marine Robert Frazure following Operation Indiana in 1966.

The small diary belonged to Vu Dinh Doan, a Vietnamese soldier who was found killed in a machine gun fight, according to defense officials. Officials said that a Marine, Robert "Ira" Frazure of Walla Walla, Wash., saw the diary — with a photo and some money inside — on the chest of the dead soldier and took it back to the U.S. 

The diary came to light earlier this year when the sister of a friend of Frazure's was doing research for a book and Frazure asked her help in returning the diary. The sister, Marge Scooter, brought the diary to the PBS television program History Detectives. 
The show then asked the Defense and State departments to help return the diary.

Reuters and The Associated Press contributed to this report. 

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