Heavy Iraqi surface-to-surface rockets blasted the Iranian city of Dezful as most of its residents slept early today, killing at least 100 persons, Iran announced.

The attack, reported by Iran's official radio and news agency, was the second major Iraqi rocket assault on Dezful, which lies astride a key oil pipeline linking Tehran with Iranian oil installations in the south. Dezful also is the site of a large, strategically important air base designed to protect the oil fields and refineries under attack by Iraqi ground forces.

Iranian officials were quoted as saying the death toll is likely to rise as rescue workers recover additional bodies from the rubble of houses destroyed by the seven 35-foot rockets said to have crashed down on the city shortly after midnight.

"Three districts of Dezful have been badly damaged," the Iranian news agency Pars said, calling the Iraqi attackers "enemies of humanity."

The first Iraqi attack, on Oct. 8, killed more than 150 persons by Iranian count when three rockets were launched on the city. Dezful also has come under several bombing attacks by Iraqi Migs since the Persian Gulf war broke out 35 days ago. But according to Iranian officials the air base has been only lightly damaged.

Both rocket attacks, however, have caused heavy damage to civilian areas, Iranian reports said. On both occasions, Iraqi gunners were said to have used 1-ton Frog 7 missiles with a range of about 37 miles, just under the distance from the Iranian border to Dezful.

Iraq possessed 26 of the Soviet-made missiles before the war along with a dozen Scud surface-to-surface missiles, a larger weapon also made by the Soviet Union.

The official Iraqi radio warned Dezful residents several weeks ago, after the first rocketing, to evacuate the city to avoid loss of civilian life in more attacks that the radio said were likely.

The Iranian president, Abol Hassan Bani-Sadr, was reported by Tehran radio to have visited Dezful after today's attack to praise residents' courage. After a tour of the damaged area, he vowed vengeance against Iraq, Pars said.

Elsewhere in the war, Iraq reported two U.S.-made Iranian Phantom jets were downed today. According to the official reports, one of the Phantoms was shot down in an early-morning dogfight with Soviet-made Iraqi Migs near the embattled Iranian cities of Khorramshahr and Abadan on the Shatt-al-Arab waterway dividing Iran and Iraq at the head of the Persian Gulf.

There were conflicting reports about the level of fighting in those cities. Iran claimed once again to have pushed Iraqis from Khorramshahr, which Baghdad said Friday it had captured after a month-long siege. But Iraqi accounts said Iraqi troops had pushed back an attempt by Iranian defenders to recapture the Karun River bridge at Khorramshahr, an important acces route for Iraqi troops moving toward Abadan 10 miles down the Shatt-al-Arab.

A communique issued in Baghdad said two Iranian patrol boats were sunk in the waterway during the day's fighting for Abadan.

In Tehran, the Pars agency reported that two Iranians believed to be Arabs were executed on conviction by a revolutionary Islamic court of spying, collaborating with invading Iraqi forces and distributing arms to "counterrevolutionaries in southern Iran."

Iran's southern province, Khuzestan, is the war's main battleground and long has been the site of autonomy activity by ethnic Arabs who chafe under the Persian central government in Tehran. Iraq, which calls the province Arabestan, has called for recognition of Arab rights, but there have been no reports of widespread support for invading Iraqi troops among the province's Arabs.