Iran accused Iraq of shooting down a commercial airliner yesterday, killing 46 persons, including several high officials, as Iranian troops reinforced their bridgehead on the Iraqi side of the mouth of the strategic Shatt al Arab waterway at the head of the Persian Gulf.

Sources with access to satellite photographs said Iran now has 30,000 troops at Faw, a former Iraqi oil port on the gulf, and has used pontoon bridges to bring in heavy weapons, including tanks and artillery, Washington Post correspondent Jonathan C. Randal reported from Paris.

The heavy reinforcement at Faw raised questions about Iran's immediate intentions. Western analysts originally had seen the Iranian push there as a likely feint, to be followed by a major offensive to the north, where large numbers of troops also have been massed. But Iran's movements in the past two days have led to speculation about a possible two-pronged move on Basra, Iraq's second-largest city.

Iranian replacements were reported arriving regularly at Faw to make up for the heavy casualties the first wave of infantry sustained in the 11 days of renewed fighting in the 5 1/2-year-old war.

Iran's official IRNA news agency said Iraqi warplanes shot down a commercial F27 Fokker Friendship airliner near Ahvaz, about 60 miles northeast of the Iraqi border, killing all 46 people aboard, including eight members of Iran's parliament and Hojatoleslam Fadlollah Mahallati, Ayatollah Ruhollah Khomeini's liaison with the Revolutionary Guards.

The agency said one of the members of parliament killed in the crash was Hassan Shah-Cheraghi, head of the Kayhan group of newspapers, Reuter reported from Bahrain.

Iraq said it had shot down an Iranian C130 transport near Ahvaz, but it said Tehran's charge that it had shot down a Fokker passenger plane was a "lie."

An Iraqi military spokesman said that "the Air Force action was limited only to striking military targets in the southern sector of Iran and in the battlefields," United Press International reported from Baghdad.

Tehran said the plane was attacked while on a flight from Tehran to Ahvaz and crashed at Veys, about 10 miles northeast of Ahvaz.

At the United Nations, Iranian Ambassador Said Rajaie Khorassani denied reports that the plane was carrying wounded from the battle front. "We don't carry the wounded from Tehran to Ahvaz. The course is usually the reverse," he said, according to Reuter.

Iraq's Air Force, which outnumbers Iran's by about 4 to 1, had played little role in the early days of this offensive but has carried out heavy bombing in the past two days. Iraq's news agency said that Iraqi planes flew 750 missions on Wednesday, the highest reported one-day total since the war began in September 1980.

Iraq said Wednesday that it had killed more than 35,000 Iranians this month, and Iran said it had killed or wounded 15,000 Iraqis. Both sides frequently have exaggerated their claims.

The sources with access to satellite photographs said the pictures indicated that in addition to putting 30,000 troops on their bridgehead at the Iraqi side of the mouth of the Shatt al Arab waterway, Iran had made a small advance to the west of Faw on the Khaur Abdallah inlet.

The inlet, which leads from the gulf to Iraq's chief naval base in the area, at Umm Qasr, also marks the Iraqi border with Kuwait which has attempted to keep out of the fighting on its flank.

Elsewhere on the front, the Iranians reportedly remained dug into strong defensive positions, the sources said, with both armies operating from trenches.

The main battle front appeared to be stabilized about 10 miles north of Faw, which was Iraq's chief oil port until the early days of the war.

The sources expressed surprise at the Iraqi Air Force's inability so far to destroy Iran's main pontoon bridge across the Shatt al Arab. The current reportedly is so strong at that point that the bridge at times is bent into a U-shape.

Meanwhile, The Associated Press reported from southern Majnoon island, 35 miles northeast of Basra, that Iraq had recaptured the Majnoon oilfield, seized by Iran two years ago.

Col. Abbas Hawi, commander of the Iraqi troops that retook the area last week, told AP that Iraq now controls most of the 40-square-mile area, including four major oil wells whose proven reserves are estimated at about 8.5 billion barrels.

The smaller northern Majnoon island, which has no oil, remains under Iranian occupation, Hawi said. It is separated from the Iraqi-held territory by less than a mile of marshland.