Iran appeared to have stiffened its resistance to the Iraqi ground offensive yesterday as warplanes of the two countries continued to pound each other's vital oil installations throughout the fourth day of their war.

But Iraq claimed to have captured a key Iranian port in the Shatt-al-Arab estuary after three days of fighting, taken 100 square miles of Iranian and territory and to have pushed at one point almost 45 miles into Iran. There was no confirmation of these claims.

In Paris, Iraq's vice premier, Tariq Aziz, announced that his country had now taken back all of the territory it claims from Iran and listed four general preconditions for the end to the fighting.

These were the end of all aggressive Iranian activities against Iraq, agreement on good neighborly relations between the two nations, noninterference in the affairs of other countries and Iranian recognition of Iraq's sovereignty over all the territories and waters it lays claim to.

Aziz indicated that the return to Arab control of three small islands near the Hormuz Strait that Iran seized in 1971 was among Iraqi war objectives but not a precondition for an armistice. He also appeared to rule out the use of Iraqi force to take them back, although Iraqi diplomats in other world capitals have been taking a harder line and insisting the islands must be returned.

Except for shipments from the two warring nations, oil continued to flow uninterrupted yesterday through the Hormuz Strait, the strategic 35-mile-wide waterway through which pass tankers carrying about half of the West's import needs. There has been considerable international concern that the fighting might spread and block the strait if Iraq tried to seize the three disputed islands.

Pakistan and Libya were the first two Arab countries to offer their mediation in the conflict yesterday, but Iran again rejected any such offers until Iraq withdraws its forces from Iranian territory and ceases its "aggression."

Fighting continued yesterday on the land and in the air and sea, with Iranian jets returning to attack Baghdad and flying all the way, across the country to bomb a gas refinery at Ain Zalah, 280 miles northeast of the Iraqi capital.

Eyewitness accounts said the Iranian planes hit the capital twice early yesterday morning, bombing an Iraqi military base as well as the Dora refinery, one of the country's biggest. There was reportedly little damage to the oil installation, however.

There was also an alert in Tehran and air raid sirens and bursts of antiaircraft fire spread fears of another Iraqi attack, before the military command announced it was only a practice run.

In the ground war, Iraq claimed it had captured the Iranian port of Khorramshahr in the disputed Shatt-al-Arab estuary after three days of hard fighting, but Iran denied it and Reuter news agency quoted a spokesman in the beleaguered town as saying, "no Iraqi troops have entered Khorramshahr."

Baghdad radio said Khorramshahr had become "the graveyard of the Persians."

Iraq also reported the taking of another town in the central sector along the 300-mile-long border front, Naft-e-Shah, making it the sixth town it claims to have captured so far. Altogether, the Iraqis now say they have occupied over 100 square miles of Iranian territory.

Iraq's war bulletins also said its troops had cut Tehran's rail link with Abadan and Khorramshahr and reached the approaches to Dezful, all towns in Iran's oil-rich Khuzestan Province where there is an Arab majority and an area Iraq has also claimed in the past.

Baghdad radio said the border between the southern Iraqi town of Basra and Khorramshahr was now "completely safe." Dezful, the town Iraqi forces were said to be nearing, is 45 miles inside Iran. It was not clear, however, how deep inside Iran the Iraqi forces were in their march on the town.

But the Iraqis appeared to be running into stiffer resistance. Iran reported it had stopped the Iraqi advance into Khuzestan Province and pushed back Iraqi forces around Qasre-Shirin at the northern end of the front, capturing 40 Iraqi tanks.

The Iranian military command also reported another 50 Iraqi tanks "smashed" after a boat ferrying them across the Shatt-al-Arab "became bogged down in mud" and ran aground. It said another 50 Iraqi tanks were knocked out in fighting around Shalamshah, an Iraqi border post near Khorramshahr the Iranians claim to have captured.

Altogether, the two armies appeared from the official war bulletins to be locked in heavy fighting at various points along the jagged 300-mile front. U.S. military specialists following the war believed yesterday that the main Iraqi objective in the ground war was to capture a small strip of territory inside Iran, in addition to the Shatt-al-Arab, which it would later offer to swap in return for Iranian recognition of Iraqi sovereignty over the waterway.

However, the Iraqi offensive into Khuzestan now seems to be aimed at taking more than just a small strip of border land.

In the air war, Iraq claimed to have shot down at least 19 more American-made Iranian jets, and Iraqi Vice Premier Aziz said in Paris that Iran had now lost 140 warplanes.

Iran, for its part, announced that it shot down another 16 Iraqi Migs yesterday, bringing the total to 70 for the four days of the war.

Iranian jets in addition to twice bombing Baghdad raided Kirkuk, Mosul and Irbil and set afire the gas refinery at Ain Zalah, the deepest point they have penetrated inside Iraq.

Iraqi jets attacked seven towns and also once again bombed Kharg Island, 123 miles southeast of the Shatt-al-Arab where Iran's main oil-loading facilities are located. There were no reports of serious damage to the facilities there, which Iran closed down Wednesday following the first Iraqi bombing.

There were widely conflicting reports as to just how much damage either the Iraqi or the Iranian jets were inflicting on the industrial sites of the other country, which have become a prime target of the war. Some reports suggested considerable destruction but others suggested it was limited compared to the impression left by the two countries' war communiques.

U.S. military specialists also were skeptical that the raiding Iranian and Iraqi fighters were inflicting very much damage on military installations. a"A lot of the raids have more pyschological than real military value," said one.

In the smaller sea war, Iraqi gunboats and helicopter gunships reportedly fought off an Iranian naval attack on the Iraqi oil port of Khor Abdulla. Iraq claimed that three Iranian frigates and two gunboats were sunk.

On the diplomatic front, Iraq said it had won the support of Saudi Arabia for its war against Iran. King Khalid was reported as saying his kingdom stood by the side of Iraq "in its pan-Arab battle and conflict with the Persians, the enemies of the Arab nation," following a telephone conversation with Iraqi President Saddam Hussein.

However, the official Saudi press agency gave a different version, saying the king had simply told Saddam Hussein of his "concern" over the war.

The war yesterday stirred Iranian exile leaders to rally to the defense of their homeland. In Cario, the exiled Crown Prince Rexa Pahlavi sent a message to the Iranian army chief of staff offering "my life's blood" to "safeguard Iran's independence and territorial integrity."

In Paris, former Iranian prime minister Shahpour Bakhtiar told the French state radio that Ayatollah Ruhollah Khomeini, the Iranian revolutionary leader, was "the root cause" of the war. But Bakhtiar said he could only be on Iran's side and would never give up an inch of territory.

In Cairo, Egyptian President Anwar Sadat said in an interview with John Wallach of the Hearst Newspapers that the war provided an excellent opportunity for the Iranian Army to overthrow the ayatollah and urged the United States to back it. "For God's sake, be viligant this time and don't lose the opportunity or give it to the Soviets Union without having to pay a heavy price," Sadat told Wallach.

In Tehran, Khomeini personally tried to reassure Iranians that "there is no problem to worry about." In a broadcast speech, he added: "We are powerful. We will defeat them. The Iranian nation must not be afraid.

"Iraq is nothing," Khomeini said. "God willing, these roots of corruption will be eradicated by the powerful hands of the nation, and Islam will spread everywhere."