WASHINGTON (AP) _ The State Department on Monday called for the return of the remains of five Americans who were killed when their U.S. government relief agency plane was downed in a missile attack over Mauritania.

Department spokesman Charles E. Redman said the ''tragic event'' underscored the importance of bringing a peaceful end to conflict in the Western Sahara.

The Soviet-armed Polisario Front, which has been fighting a guerrilla war since 1975 for control of the region, accepted responsibility for the downing last Thursday of a private DC-7 aircraft under contract to the U.S. Agency for International Development. The Polisario also apologized and expressed regret.

Redman withheld judgment until U.S. investigators in Morocco questioned crew members of a second plane, which landed at Sidi Ifni after reportedly being hit by missiles.

Redman said the Americans' only mission was to eradicate locusts by sparying a pesticide. The planes had been leased from T&G Aviation of Chandler, Ariz. John Wiliamson, a special assistant to the AID, said last week they had no connection to any American intelligence operation.

''Our must urgent priority at this time,'' Redman said, ''is the recovery and return of the remains of our fellow citizens to their families in the United States. We are in urgent consultation with all parties who might help in this effort.''

The U.S. official said the crash site was in a remote area of the Western Sahara controlled by the Polisario. ''We don't know, at this point, the exact circumstances.'' Redman said.

The guerrillas and the Moroccan government have been holding indirect peace talks under the sponsorship of the United Nations. Redman appealed for cooperation in the effort.