Iran's newly launched offensive against Iraq in the oil province of Khuzestan is beginning to look as big and bloody as the March attack that decimated three Iraqi divisions, U.S. officials said yesterday.

They added that Syria has become a new and major player in the war by supplying an unprecedented amount of arms to Iran within the last 30 days, including 90 field guns of varying size. North Korea remains the main supplier for Iran, however, they added.

U.S. intelligence agencies say at least two divisions are already engaged on each side in the fighting swirling around the port city of Khuninshahr (formerly called Khorramshahr) and the town of Ahwaz on the Karun River. Iran announced last Friday that it had launched the new offensive in that region, but U.S. officials said it has been building ever since and featured a big night attack this past Wednesday.

In a communique from the Iraqi high command, broadcast by the state-run radio, Iraq said its jets and helicopter gunships blasted Iranian positions in 96 bombing runs Thursday west of the Karun River, supporting a ground counteroffensive, The Associated Press reported. Baghdad said Iranian forces at the Taheri and Haloub beachheads west of the Karun were "fleeing in chaos" eastward and said one Iranian jet fighter and four helicopter gunships were downed, and eight tanks were destroyed in this area.

Iranian communiques said enemy counterattacks were all shaken off, with heavy Iraqi casualties, and said waves of rocket-firing Iranian helicopters raided an Iraqi border town and Iraqi Army headquarters at Chalamcheh, AP reported. There was no independent confirmation of the claims.

The big worry for other Persian Gulf countries and the United States, one U.S. analyst said yesterday, is that Iranian forces will be so emboldened by recent successes that they will take the war into Iraq proper, heightening the danger of conflict all around the gulf.

Officials said that Syria's entry as a big supplier of arms for Iran is also a worrisome development. One analyst said Washington suspects, but cannot prove yet, that the Soviet Union is sending the arms to Iran with only a short laundering stop in Syria. The Soviets at the same time are still supplying Iraq with limited quantities of its older weaponry, including T54 and T55 tanks, in this odd, on-again, off-again war that broke out in September 1980.

Asked why Moscow would be sending arms to both sides at this point, one official speculated: "They probably want to end up being the broker for both sides as they were for the Indians and Pakistanis" in the Indo-Pakistani war of 1971.

U.S. officials expressed amazement at the ability of Iran to launch this new offensive so soon after the March one that inflicted heavy casualties on both sides. They also said that the Iranian military has perfected a pattern of ground attack that the Iraqis have not yet been able to master.

Both in the March offensive and the one launched a few days ago, the pattern was the same, according to U.S. intelligence. The Iranians sent in waves of infantry at night that ran through and past Iraqi forward positions and disrupted command and control centers a few miles to the rear of the front line. Then, at first light, Iranian artillery, helicopter gunships and armor blasted away at Iraqis caught between the newly formed forward line and the old one as they rushed to seize key roads and waterways.

Once the Iranians hold vital roads and waterways, it becomes difficult for the Iraqis to reinforce their own forward line with supplies and troops to replace those wounded or killed. By the Pentagon's count, Iran launched three major offensives before this latest one, making it the fourth and perhaps most decisive. U.S. analysts said they expected the Iraqi command to counterattack rather than yield the territory around Khuninshahr.

While admitting that their figures are far from exact, U.S. officials said that it appears that about 15,000 soldiers have been killed on each side so far in the Iranian-Iraqi war. They estimated that about 8,000 Iraqi prisoners are in Iran now but said they had no count for the number of Iranian prisoners held by Iraq.