The announcement from Vietnam Minister of Defense Phung Quang Thanh comes as U.S. Secretary of Defense Leon Panetta and his Vietnamese counterpart participated in a first-of-its-kind joint exchange of artifacts from the war in Hanoi.

A Department of Defense spokesman said in a statement the department believes Joint Prisoners of War, Missing in Action Accounting Command (JPAC) research teams will greatly benefit from access to the new sites in their search for the approximately 1,200 U.S. service members still missing in Vietnam.

"The Department of Defense believes these sites are critical to locating missing-in-action troops from the Vietnam War," spokesman George Little said.

In the artifact exchange, Panetta gave his Vietnamese counterpart the Vũ Ðình Ðoàn diary, which was taken by Robert Frazure, United States Marine Corps, following Operation Indiana in 1966.

In turn, Quang Thanh presented personal letters of U.S. Army Sgt. Steve Flaherty, who was killed in action in 1969.

The following are quotes from the four Sgt. Flaherty letters received today.

“I’m sorry for not writing so long but we have been in a fierce fight with N.V.A.," he writes to a person identified as Betty. "We took in lots of casualties and death.  It has been trying days for me and my men.  We dragged more bodies of dead and wounded than I can ever want to forget.”

Both leaders agreed to return the artifacts to the relatives of the soldiers.

During a press briefing, where the two defense chiefs formally handed over the papers, 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.

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.

June 4, 2012: Vietnamese Minister of Defense Phuong Quang Thanh (R) presents the personal letters of US Army Sergeant Steve Flaherty, who was killed in action in 1969, as well as other letter of unknown US Soldiers, to US Secretary of Defense Leon Panetta during a press conference at the ministry in Hanoi, Vietnam.

"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.

During the meeting with Panetta, Vietnamese officials said they would open the three previously restricted sites that the Pentagon believes are critical to locating troops missing in action.

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, in many cases leaving 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.

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.

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.

In a letter to "Mother," Flaherty writes: “We couldn’t retrieve the bodies of our men or ruck sacks and when we brought air strikes, jets dropped napalm and explosives that destroyed everything that was there. ... I definitely will take R&R, I don’t care where so long as I get a rest, which I need so badly, soon. I’ll let you know exact date. ... If Dad calls, tell him I got too close to being dead but I’m O.K.  I was real lucky.  I’ll write again soon.”

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.

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.

In a letter to “Mom,” Flaherty writes: “Our platoon started off with 35 men but winded up with 19 men when it was over. We lost platoon leader and whole squad. ... The NVA soldiers fought until they died and one even booby trapped himself and when we approached him, he blew himself up and took two of our men with him.”

In a letter to “Mrs. Wyatt," Flaherty writes: "Our company and Alpha Company lost a total of 50 men in fierce fight. ... Our platoon leader was killed and I was the temporary platoon leader until we got the replacement. Nothing seems to go well for us but we’ll take that ridge line. ... This is a dirty and cruel war but I’m sure people will understand the purpose of this war even though many of us might not agree.”

Fox News' Jennifer Griffin and The Associated Press contributed to this report.