Retreating Iranian soldiers were reported today to have knocked out a key bridge over the Karun River at the port city of Khorramshahr just before Iraqi invaders could storm across it.

The bridge was a crucial prize for Iraqi forces advancing around Khorramshahr because it would have facilitated their entry into the city itself, which has been under siege for four weeks but which is being fiercely defended by die-hard Iranian Revolutionary Guards. Iraqi sources here insisted, however, that its destruction would not stop them.

In the air war, a pair of Iran's U.S.-made F4 Phantom fighter-bombers struck at the Iraqi capital of Baghdad, wounding eight civilians, and one was shot down by a surface-to-air missile, an Iraqi communique reported. Other Iranian planes attacked the northern Iraqi resort town of Haj Omram, it added, while Iraqi Migs struck back at targets near the Khuzestan Province cities of Ahwaz and Dezful.

The Iraqi defense minister, Adnan Khairallah, said today in an interview with the official Iraqi press that his Army has captured an area of Iran approximately equal to that of New Jersey, most of it in the oil-rich Khuzestan region. An Iraqi first deputy prime minister, Taha Yassin Ramadan, told the French newspaper Le Monde that Iraq will try to take all the Khuzestan oilfields and use them as leverage in whatever negotiations grow from the war.

It was clear from positions across the Shatt-al-Arab waterway from Khorramshahr, however, that the Iranians were still putting up a staunch defense.

For one thing, soldiers barred the way to a small island in the middle of the Shatt, called Mother of Bullets, from which coorespondents as recently as two days ago were able to look across into Khorranshahr. The Iraqi soldiers said Iranian small arms fire occasionally raked the island, which was now considered unsafe.

An official Iraqi communique reported that the island was hit in one of four Iranian air raids on this area yesterday. An Iranian plane, undeterred by any Iraqi antiaircraft fire, came over near the command post leading to the island this morning strafing the area while newsmen were talking to soldiers. Further down the road, there was an orange flash that looked as if a bomb had hit Iraqi territory.

Iraq said 10 civilians died and 23 were injured in yesterday's air raids. Near here, soldiers said, the houses had been abandoned because of the fighting and several fishing boats moored in the Shatt were destroyed.

The sound of Iranian mortars pounding over the Shatt into Khorramshahr could be heard clearly in the background from here with no answering fire from Iranian artillery.

The Iraqi soldiers said Iranians had blown up the Khorranshahr bridge this morning with artillery from a naval base in the narrow, 2 1/2-mile-wide belt along the Shatt that is all they still hold of Khorranshahr, which was once Iran's major port. This belt stretches along the Shatt from the east bank of the Karun River to Abadan, the site of the world's largest oil refinery, which is also under fire from the Iraqis. o

Iranian radio today said the fierce battle for Khorramshahr was continuing with hand-to-hand fighting in the streets, but acknowledged that Iraq had captured the "prison quarter" of the town. This marked the first Iranian admission that Iraqi forces have taken control over positions inside the city. a

The official Iranian news agency, Pars, said fierce fighting took place today east and south of Abadan, particularly at the Bahmanshir River bridge that forms an entrance into the city. But Iran also said there was a lull in the Iraqi artillery barrage into Abadan from huge batteries stationed near here. Traveling down the road, there were no signs that the batteries were firing this morning.

Nonetheless, from the road all that could be seen of Khorramshahr were nine big plumes of black smoke curling up to the sky.

There were signs that the Iranian air raids were taking their toll of the civilian population in these Iraqi towns on the Shatt.

Al Faw, a prosperious oil terminal town at the mouth of the Shatt, was reported today by Iraqi residents to be nearly empty as many people who live there had fled to safer refuge elsewhere. The same was reported to be true of Seiba, a town across the Shatt from Abadan.

On the road today, truckloads of refugees could be seen heading toward Basra, a major Iraqi port city which has emerged relatively unscathed from any Iranian air raids.

Although Basra got its regular noontime air raid early in the month-old war, Iranian Phantom jets have stopped coming over this city even though air raid sirens sound at least three times a day. They are generally ignored by the population except at night, when all the electricity in Basra is turned off.

As the fighting ground on, Secretary General Habib Chatti of the Islamic Conference reported mixed findings in his efforts to mediate an end to the conflict. Iraq, he said on his arrival in Pakistan, has agreed "in principle" to receive a peace mission of seven Islamic heads of state. But the Iranian leader, Ayatollah Ruhollah Khomeini, has refused his agreement so far, he added after visits to Tehran and Baghdad.