United States Defence Secretary Leon Panetta has visited Vietnam as part of a week-long trip around the Asia-Pacific. He the most senior US official to visit since the end of the Vietnam war in 1975.
The Vietnamese government said on Monday it would open three new sites for the excavation of the remains of American soldiers.
The sites were previously closed.
They are in in Quang Binh province in central Vietnam, in Kon Tum province near the borders with Laos and Cambodia, and in Quang Tri province.
Mr Panetta is in the region to explain Washington's new military strategy, which will see 60% of US warships deployed in the region by 2020, compared with 50% now.
A memorandum of understanding between the US and Vietnam was signed last year on defence cooperation.