Telephone communications to about 35 towns in El Salvador have been severed by fighting between government troops and leftist guerrillas that claimed dozens more lives, authorities said yesterday.

The Army claimed it killed 15 guerrillas in a raid on a rebel outpost near the town of El Paisnal, 15 miles north of the capital. Two soldiers were reported killed.

In San Salvador itself, officials said they found the corpses of eight people, presumably shot to death by troops enforcing a dusk-to-dawn curfew imposed last week by the U.S.-backed ruling junta.

Spokesmen for the junta claim a leftist guerrilla "general offensive" launched Jan. 10 was "totally crushed." The defense minister, Col. Jose Guillermo Garcia, said late Monday that the armed forces killed about 1,000 guerrillas since Jan. 10, while losing only 97 government troops.

Three members of the U.S. Congress returning from a 10-day factfinding trip to Central America reported alleged atrocities by Salvadoran troops against civilians and called for an immediate halt in U.S. military aid.

In Miami, Rep. Gerry E. Studds, [D-Mass.], said U.S. Ambassador to El Salvador Robert White told him that no probe into the killings of three American nuns and an American lay-worker was under way. Studds and Reps. Barbara A. Mikulski [D-Md.] and Robert W. Edgar, [D-Penn.] sent their findings to both the outgoing Carter administration and the new Reagan administration.