Iranian tanks and infantry launched a major attack against entrenched Iraqi forces yesterday, according to U.S. intelligence officials, who said Iran's "human wave" offensives were leaving extremely heavy casualties on both sides.

Yesterday's assault, a new phase in the offensive begun by Iran Monday, so far appeared to leave neither side with the upper hand, the U.S. officials said. The Iranians overran one Iraqi battalion earlier in the offensive, but this was not viewed here as decisive given the large numbers of troops committed on both sides, the officials said.

Iran claims to have retaken 120 square miles of its territory near the battlefront along the south-central part of the two nations' border, while Baghdad claims that 15,000 Iranians have been slain. Adding to signs that the renewed fighting could be spreading, Baghdad said yesterday that its Air Force had left three Iranian ships burning and sinking at the entrance to a waterway leading to the Persian Gulf.

The Iranian offensive, among the biggest in the 28-month-old war, began before dawn Monday on the plains between Dezful, at the foot of the Zagros Mountains 300 miles southwest of Tehran, and the Iraqi provincial capital of Amarah, 200 miles southeast of Baghdad. U.S. officials estimated the forces involved at 100,000 on each side.

The Iranians attacked at both ends of a front estimated at 20 to 40 miles wide, but these first attacks apparently were feints designed to draw the massed Iraqi forces away from the center.

"There is a major attack under way in the center of the line," said a U.S. official yesterday. According to information reaching here, the Iranians threw "human waves" of infantry against the Iraqi center before committing the main body of their armor to the attack.

The commander of the defending Iraqi 4th Army Corps, Maj. Gen. Hisham Sabah Fakhri, said that a map found on a prisoner showed that the Iranians' main objective was Amarah. The city is important strategically because it straddles the main highway to Baghdad from the port of Basrah and Kuwait, a key supply route for the Iraqis.

Baghdad radio said that the Iranians attempted to drive across the border yesterday morning for the third consecutive day, but "the attack was completely crushed" in 10 hours of fighting. Three earlier Iranian attacks had been beaten back, with "the enemy being unable to capture even a single inch of territory," Fakhri declared in a news conference at his headquarters.

Tehran radio, which claimed on Monday that 100 square miles of Iranian territory had been liberated and added another 20 square miles on Tuesday, claimed no new advances yesterday. Iranian communiques have announced the capture of Iraqi fortified positions at Fakeh, near the border east of Amarah. Foreign correspondents have not been allowed to visit the front on either side during the current offensive.

It has taken three days for officials here to assess the new fighting, in part because rain and cloud cover on Monday obstructed picture-taking by U.S. reconnaissance satellites on the first day of the battle.

Fakhri, the Iraqi field commander, said the Iranians had thrown six divisions, including many teen-aged Revolutionary Guards, against his forces.

In the air war, so far largely controlled by Iraq, Baghdad radio said Iraqi jets and helicopter gunships made 218 bombing raids yesterday and returned safely to their bases. An earlier Iraqi communique, claiming that three Iranian "naval targets" were destroyed at the north end of the Persian Gulf, did not elaborate on the types of ships attacked.

Iran's official news agency made no mention of any attack on shipping but said that the Iranians shot down four Iraqi planes.

There was no word of fighting along other parts of the 625-mile-long front. Last summer the Iranians staged a major attack near Basra at the southern end of the border and in the autumn staged assaults at Mandali, the border point closest to Baghdad, and near Amarah.

On the diplomatic front, Egypt and Jordan have reaffirmed their support for the Iraqis, who are backed by most Arab nations except Syria and Libya. The Iraqi news agency said Egyptian Minister of State Boutros Ghali carried a message of support from President Hosni Mubarak to Iraqi President Saddam Hussein in Baghdad last night. In Amman, the Jordanian Cabinet issued a statement calling on all Arab nations to stand united behind Iraq.