Salvadorans say 2 planes from nearby lands bagged

Salvadoran fighter jets shot down a Nicaraguan Air Force plane and a small Costa Rican plane that was air-lifting guns to rebel forces in El Salvador, military sources say. The air strikes were the first reported incidents involving a Nicaraguan Air Force plane in the 17-day leftist offensive against El Salvador's US-backed junta. Neither Nicaragua nor Costa Rica had any immediate comment.

The Reagan administration is expected to cancel all US aid to Nicaragua if firm evidence is found to support allegations that Nicaragua's gov ernment is aiding the guerillas.