Iranian reinforcements, including troops and materiel, poured into their bridgehead around the Persian Gulf port of Faw today despite Iraqi efforts to blunt the invaders' drive, according to western sources with access to observation satellite intelligence.

Even if an all-out Iraqi counteroffensive succeeds in breaking up the Iranian advance northward to Basra, Iraq's second-largest city, military analysts believe that the Iraqis can do so only at the price of diverting key units from the crucial sector north of Basra.

The sources reported huge concentrations of Iranian forces in the area east of Majnoon Islands, where Tehran has launched major offensives in the past.

Military analysts studying the photographs suggested that the Iranians appeared prepared to go into action in that area, north of Basra along the flooded Hawizah marshes, possibly within 24 hours. But the four-day-old Iranian drive at the mouth of the Shatt al Arab estuary now strikes analysts as a major thrust rather than a feint, as had been thought before today's satellite observations.

The satellite photographs were said to show Iranian materiel moving across the Shatt al Arab by boat and over pontoon bridges into the bridgehead in the Faw Peninsula.

Iranian military communiques said their forces were advancing northward, apparently through palm groves that line the western shore of the Shatt al Arab, "liberating more territory."

Iranian military communiques also accused Iraq of resorting once again to chemical weapons in a "desperate bid to halt the advance."

In Washington, State Department spokesman Charles Redman said that the United States could not confirm charges by both Iraq and Iran that the other side had used chemical weapons. He said that there was some evidence earlier that Iran was developing a capacity to produce such weapons and that Washington already had taken steps to impose "rigid controls" on the export to Iran and Iraq of materials used in their manufacture.

[One official said that it was "quite possible" that such weapons were being used but added that it probably would take several days before this could be determined.]

Military analysts were struck by the apparent failure of the Iraqi Air Force -- which outnumbers its foe 4-to-1 -- to smash the Iranian resupply effort.

U.S. analysts in Washington expressed "surprise" that Iraqi forces had not been able to dislodge the several thousand Iranians at Faw. They said the Iraqis appeared to have been caught off guard by the Iranian thrust from the south.

["They weren't expecting it," one remarked. They said the Iraqis appeared to be having trouble coping with the Iranian tactic of reinforcing their position at Faw by the use of simple pontoon bridges and small boats during the night.]

Although some sources said Iraqi warplanes had been active during daylight around Faw and north of Basra, the Air Force had been unable to stop the Iranians' resupply effort by night.

In previous offensives, the Iranians have failed to provide the necessary logistical support for troops who succeeded in crossing the Tigris River farther north.

Last March, Iranian troops who managed to cross the marshes and seize stretches of the vital Baghdad-Basra highway, eventually were cut to pieces by Iraqi helicopter gunships and armor.

The Iraqi high command announced this morning that its troops had begun a counteroffensive down the peninsula from Basra.

A 7th Army commander was quoted as saying, "The final destruction of the Iranian troops is being carefully implemented" and that they were trapped in an "area of death."

Quoting diplomats in Baghdad, The Associated Press reported that a crack Iraqi commando force had managed to punch through Iranian lines and was advancing "inch by inch" across palm groves along the Shatt al Arab.

The diplomats said a southern pocket of Iranian troops was cut off from the Iranian mainland and under continuous dawn-to-dusk attack by Iraqi helicopter gunships.

During the day, Iraqi President Saddam Hussein paid one of his infrequent visits to the front, where, according to reports from Baghdad, he was briefed on the 7th Army's strategy to "drive out the last aggressor from Iraqi soil."

Iranian military communiques claimed that their troops had captured 150 more Iraqi prisoners -- bringing the total since Monday to 1,400 -- and had shot down more Iraqi planes, making this week's total 21.

The satellite observations also supported Iranian claims that their troops had pushed west from Faw and had reached the eastern shore of the Khaur Abdallah waterway opposite Kuwaiti territory. But the satellite photographs showed that the Iranians were apparently still a good way from the Iraqi naval base at Umm Qasr on the gulf.