BANGKOK, Thailand (AP) _ Vietnam's foreign minister has pledged to try to resolve by the end of the year 70 cases of missing U.S. servicemen that the United States considers of highest priority, Voice of Vietnam said today.

Minister Nguyen Co Thach was quoted as saying he welcomed U.S. participation in joint excavations for the remains of missing servicemen.

The United States is trying to account for nearly 1,800 Americans missing in action from the Vietnam war, which ended in April 1975 with a communist victory over the U.S.-backed South Vietnamese government.

U.S. forces lost 58,000 soldiers during the war.

The radio broadcast, monitored in Bangkok, said Thach wrote a letter July 9 to retired Gen. John W. Vessey, a special presidential envoy, in which he urged the United States to ''carry out all agreements relating to humanitarian issues relating to Vietnam.''

The letter apparently referred to an August 1987 agreement between Thach and Vessey in which Vietnam agreed to accelerate the search for MIA remains. In return, Washington sent in medical teams and encouraged private agencies to help the former enemy with medical problems, including rehabilitation of veterans disabled in the war.

Hanoi has indicated it wants more U.S. help.

The official radio said Vessey welcomed Vietnam's readiness to hold joint excavations for remains. It said that in a letter Monday, Vessey said he hoped cooperation between the two countries could be expanded and ''also affirmed that the U.S. side will continue trying to respond to Vietnam's humanitarian concerns.''

In August 1987, Vessey gave Thach a list of 70 cases of missing U.S. servcemen, and he asked Vietnam to pursue them on a priority basis. Washington says that in these so-called ''discrepancy cases,'' there is strong evidence the Vietnamese know what happened to the missing men.

Voice of Vietnam said: ''Vietnam's Foreign Minister Nguyen Co Thach affirms Vietnam's efforts to solve before the end of this year 70 cases of Americans missing in action considered priority by the U.S. side. He welcomes the U.S. side for joint efforts in seeking out and excavating the remains of MIAs in August, September and October this year.''

In late 1985, the two countries carried out a joint excavation of a U.S. B- 52 crash site to search for remains, but there have been no additional excavations despite Vietnamese pledges to hold them.

This month, Vietnam turned over what were believed to be the remains of 25 U.S. servicemen, one of the largest returns of remains since the war ended.