UNITED NATIONS (AP) _ The United States and the Soviet Union on Friday urged the U.N. secretary- general to redouble his efforts to achieve a cease-fire between the Salvadoran government and leftist guerrillas.

Secretary of State James A. Baker III and Soviet Foreign Minister Eduard A. Shevardnadze signed the letter to Secretary-General Javier Perez de Cuellar.

Western diplomats said it was the first time they could recal the United States and the Soviet Union sending a joint letter to the secretary-general about regional conflict resolution.

The letter was written before the downing of a U.S. helicopter by Salvadoran rebels on Wednesday. The U.S. administration said initial reports showed the three airmen were executed by rebels. The rebel Farabundo Marti National Liberation Front denied that claim.

The contents of the letter were not disclosed, but U.S. diplomats said it generally encouraged Perez de Cuellar and his special representative Alvaro DeSoto to push for talks in Mexico aimed at achieving a cease-fire and a political settlement.

It was not known whether the letter called for increased U.N. presence in the region. U.N. military observers in Central America are directed to prevent the territory of one country from being used to launch attacks upon another.