Vietnam has said it will open up three new areas to American teams searching for US servicemen who went missing during the Vietnam war.
The agreement was made between Vietnamese Defence Minister Phung Quang Thanh and his visiting US counterpart, Leon Panetta.
Mr Panetta was in Vietnam for a two-day visit to discuss military ties.
They also conducted an unprecedented exchange of artefacts taken by soldiers from both nations during the war.
Mr Panetta presented the diary of a Vietnamese soldier taken by US marines in 1966.
In return, Mr Quang Thanh gave the defence secretary the personal letters of a US killed in action in 1969.
US officials believe the three previously restricted areas that have now been opened up for excavation could be important in the hunt for missing servicemen.
Two of the areas, in Quang Binh and Quang Tri provinces, were the sites of suspected plane crashes during the war.
The third site in Kon Tum province, near the borders with Laos and Cambodia, was the scene of a battle in 1968.
The defence secretary said the US had an "enduring commitment" to build a strong defence partnership with Vietnam.
His visit follows the signing of a memorandum of understanding on defence co-operation between the two former enemies last year.
Correspondents say the meeting was an attempt to shore-up American power in the Asia-Pacific region in the face of growing Chinese influence.