Buoyed by its biggest victory in the 18-month-old war against Iraq, Iran is pressing its offensive, and the only major question among Iranians, as well as many foreign diplomats and military analysts, is how fast the Iranians can drive the Iraqis out of their vital southern oil fields.

Iran's leadership is now judged more determined than ever to press its military advantage and bring about its long-stated goal, the fall of the Baghdad government.

Ayatollah Ruhollah Khomeini, a diplomat said, "is out to get them all--the shah is dead, Jimmy Carter is no longer president of the United States and he wants Saddam's head."

So far the Iranian military has refrained from attacking Iraq proper, perhaps realizing that it was Saddam Hussein's invasion of Iran in September 1980 that rallied warring factions here to repulse a foreign enemy.

Influential Iranian Moslem clerics recently have suggested, however, that Iran should capture Kerbala and Najjaf, two of the holiest shrines of Shiite Islam located in southern Iraq.

Khomeini spent most of his 14-year exile in Najjaf and never forgave the Iraqis for expelling him in October 1978, just months before his followers overthrew the late shah, Mohammad Reza Pahlavi.

The major improvement in Iran's military fortunes--which has cleared the Iraqis out of almost all of Iran's central sector--has come at an auspicious time for the Islamic rulers.

The winter was a period of pressures caused by continuing meat and other food shortages that were compounded by strains caused by the war and fears of attacks from armed domestic enemies. The revolutionary leadership now faces spring with greater optimism. "The worst is over for them," a Third World diplomat said.

"Undeniable Victory," as the current offensive west of Shush is code-named in Persian, is threatening to become just that.

Quite apart from the destruction of three Iraqi divisions, the interruption of Iraqi lines of communication between troops in the north and those in the southern oil fields, the capture or destruction of about 300 tanks and armored vehicles and the regaining of hundreds of square miles of Iranian territory, the offensive's greatest achievement, according to professional analysts, is that it appears to have broken Iraqi Army morale.

"The Iraqi prisoners shown on television here don't even look tired or haggard," one such specialist said. "They didn't fight, they surrendered en masse." He attributed much of their apparent unwillingness to fight to the failure of the Iraqi high command to rotate troops in and out of the elaborate trench works and strongholds established in the sector soon after the 1980 invasion.

This impression was borne out by interviews with a handful of about 500 prisoners that are under guard at a Tehran airbase.

Part of the 15,000 or so prisoners the Iranians claim to have captured since the operation began early March 22, the men appeared relieved to be out of the war and showed few signs of combat-caused stress.

"This has been the turning point of the war," the specialist noted, "and I see no way Saddam Hussein can stiffen the back of his armored forces now . . . I do not think Iraq has enough left to throw into battle and turn the tables.

"Iraq has sustained a significant loss," he said. "When Saddam Hussein announced he was withdrawing the Fourth Army to Iraq proper, the truth was there was little for him left to withdraw."

Military analysts were especially impressed by the Iranian military's degree of coordination among the regular armed forces, the Revolutionary Guards and the so-called "Army of 20 Million" volunteer force.

In the past the armed forces, long suspect because of their privileged status under the shah, were at loggerheads with the Revolutionary Guards, whom specialists now credit with shaping up into an "almost professional paramilitary outfit."

"They've learned a lot in 18 months of fighting," an analyst said.

The offensive that began 10 days ago was described as a classic World War I operation involving reducing the mine fields and artillery protecting Iraqi positions on high ground dominating the otherwise flat expanse of desert in southern Khuzestan.

Analysts credited the Iranians with using their American-built fighter-bombers and rocket-mounted Cobra helicopter gunships to great effect against Iraqi armor. For most of the war both sides have husbanded their air forces rather than commit planes to ground support. Iraq is thought to have lost as many as 18 warplanes against perhaps only two or three for Iran, according to analysts.

By professional military standards the current operation, involving four or five Iranian divisions, normally is not considered complicated. But the Iranian performance was described as remarkable given the rivalry among their fighting forces and the purge of all general officers that the Khomeini government carried out upon seizing power.

Although the Iraqis had expected an Iranian offensive around the Iranian new year starting March 21, they apparently were taken by surprise when the fighting did not center on Khorramshahr, the Persian Gulf port, as had been advertised.

In the central sector during the last few months the Iranians had gained a foothold along the Karkheh River and established a small pocket on its west side. The Iranians massed troops in the nearby hills, then committed more than 100,000 soldiers, including four regular divisions, as well as at least 30,000 Revolutionary Guards and an equal number of volunteers from the "Army of 20 Million."

Facing them were three Iraqi divisions, which had to be reinforced by eight independent brigades. In the end the Iraqis fielded between 60,000 and 70,000 men to no avail, according to military analysts here, as the Iranians recaptured about 600 square miles of territory.

Fighting is continuing in a 200-square-mile area just north of Bostan--a town recaptured by Iran two months ago--where Iranians were said to have driven a wedge between the remaining Iraqi forces right down to the common Iraqi-Iranian border.

As has been true throughout the war, the Iranians again demonstrated their willingness to absorb high casualties, in keeping with the revolutionary Islamic government's acceptance of martyrdom.

"Undeniable Victory" followed the pattern the Iranians established last fall when they first broke the siege at the oil refining center of Abadan, then recaptured Bostan and Susangerd in the central sector without any meaningful Iraqi response.

West of Dezful, the Iraqis now reportedly have been pushed back to a line five miles inside Iran. Elsewhere, they still hold Iranian territory west of Ahwaz to the south and around Qasr-e-Shirin farther north.

Specialists are convinced that without the north-south rail and road links they had enjoyed until now the Iraqis will be hard pressed to maintain their hold on the southern oil fields.