In a new drive to capture the battered Iranian port of Khorramshahr, the Iraqi Army today was reported to have bridged the Karun River north of the city in an effort to cut off its dogged defenders from reinforcements and supplies.

The Iraqi effort to surround the besieged port that has resisted their devastating artillery assaults for 19 days came as Iran claimed to have repulsed an Iraqi special forces thrust toward the heart of the city and to have launched a counterattack to the north at Ahwaz, the capital of Iran's oil-rich Khuzestan Province.

Maj. Hamed Abdullah, the brigade commander of the Iraqi troops on the right bank of the disputed Shatt al-Arab waterway directly across from the devastated port, said today that the Iraqi Army had established a bridge-head on the east bank of the 120-yard-wide Karun River after having laid a pontoon bridge over it.

The major said that according to radio transmissions he had intercepted from the front across the river, an indeterminate number of Iraqi tanks had already crossed the makeshift bridge in an attempt to encircle the city.

If the Iraqis manage to drive south from their bridgehead to the Shatt-al-Arab along the eastern bank of the Karun, they will have finally accomplished a maneuver they initially claimed to have completed Sept. 24, two days after the war began. At that time, the Iraqi government claimed its forces had "surrounded" both Khorranshahr and the oil center at Abadan, some nine miles to the east, and that their fall was "imminent."

That was more than two weeks ago. The Iraqi government had hoped the superior fire power, manpower and discipline of its military would simply roll over the Iranian forces, sapped as they were believed to be by two years of Islamic revolutionary purges.

After almost three weeks of war, the Iraqi forces are still largely in the same positions they were in the first three days of their assault into Iran.

After moving rapidly across the sparsely populated wastelands along the Iranian border, the Iraqis have bogged down outside the urban centers that were their main objectives. Khorramshahr, Abadan, Ahwaz, Dezful and Kermanshah, to the north, all have remained in Iranian hands, stalling the Iraqi drive into a costly, sapping war of attrition.

This can hardly be what was envisaged by Iraqi President Saddam Hussein and his chief of staff, who expected Iran to be a pushover. Their plan was for lightning strikes that would roll up Khuzestan Province, the heart of Iran's oil-based economy, forcing Ayatollah Ruhollah Khomeini and his mullah-led government to sue for peace on Iraq's terms.

This has not happened. No matter what happens to the now destroyed Khorramshahr -- and given the sheer weight of superior numbers and equipment fielded by the Iraqis against the city's motley force of defenders, the city will undoubtedly fall sooner or later -- the Iraqi assault has been costly, and potentially damaging politically.

The slowness of Iraqi advances has allowed Iran to retaliate against Iraq's vital oil industry, destroying by air strikes important parts of its refineries, pumping stations, pipelines and other installations.

The forced suspension of Iraq's oil exports has meant that its loss in oil income alone from the war is costing Iraq a staggering $84 million a day or $3.5 million an hour. This does not even include the actual cost of the war itself in lost equipment, diverted labor and destroyed economic facilities.

Iran, of course, has suffered as much, if not more in economic terms. Its own oil industry is in shambles and its oil exports, though only minimal since the revolution two years ago, have also been halted because of bomb damage to its own installations.

After two years of revolutionary dislocation, however, Iran is used to living on less, with hardship and deprivation, and the economic losses are therefore felt much less.

With some four divisions totaling 70,000 troops and almost 1,000 tanks in the field in Khuzestan, the Iraqi Army has now, according to Iranian reports at least, resorted to using Soviet-built Scud surface-to-surface missiles with massive 200-pound warheads against such cities as Ahwaz and Dezful in hopes of shattering Iranian morale, which has so far failed to crumble under normal artillery attacks.

If Khorramshahr is any example, the Scud will hardly prove to be the secret weapon that turns the tide of war. While no Scuds have been fired into the port, the almost three weeks of artillery pounding have done as much damage as any Scuds, and still the Iranians are resisting.

Even if the Iraqi thrust across the Karun succeeds in isolating the port's fanatical defenders, their tenacity and effectiveness have proven that Iraq will have to pay a very heavy and perhaps ultimately insupportable price for every further foot of Iranian land that they try to take.