They call it here the "noonday Phantom" and, without fail since the war with Iran began 10 days ago, it has swooped in over this shabby and sleepy riverside city on the Shatt-al-Arab with the punctuality of the chimes of Big Ben.

There is the now too familiar screech of air raid sirens and, the few times that Iraqi radar has managed to pick up the invader, there has been the pop-pop of antiaircraft flak guns around the city. The fire is usually late, however, after the Iranian F4D Phantom has loosed its bombs at one of the city's dozen strategic targets.

Despite claims to the contrary in official communiques emanating from Baghdad, the plane invariably roars off unscathed.

Only rarely are the invading Phantoms challenged by the Soviet-made missiles that are the most sophisticated anti-aircraft weaponry in the Iraqi military arsenal.

More surprising, in the last six days, the Iraqi Air Force has never been seen.

It does not defend the vital strategic bridges and petro-chemical industries around this legendary city once known as Sinbad the Sailor's home port. Nor is the Iraqi Air Force appearing these days over Iraq's vast Army deployed in Iran's neighboring Khuzestan Province to provide the sort of air cover and ground support that is the keystone of modern warfare.

While the Iraqi Air Force has apeared around the capital of Baghdad to try to counter Iranian air strikes therre, and has made sporadic attacks on Iranian targets in recent days, it is hard when viewing its absence from in and around the battle front here to escape the conclusion that it is only just barely operational today.

Before Iraq launched its war against Iran early last week, the Iraqi Air Force was made up of 339 Soviet-made combat aircraft, including 80 sophisticated Mig23s and some 30 equally effective Su20 fighter bombs as well as Mig21s and a bomber squardon of Tu22 bombers.

How much of that force survived the initial Iranian retaliatory raids, replying to Iraqi attacks on 10 Iranian air fields -- including the military side of Tehran's Merhabad International Airport -- is still a subject of conjecture here.

However, the Iranian retaliation in the first two days of the war was fierce. Foreign observers who witnessed their attacks around the Iraqi air base west of the oil refinery town of Shueiba near here report fierce dog-fights in the air, streaking SAM missile trails, strafing and bombing and, according to one foreigner who witnessed it, the downing of at least eight Iraqi Migs.

Since those attacks, operations at the strategic Shueiba Air Base, the closest to the Khuzestan front where the Iraqi Army has made its main thrust into Iran has been limited. Foreigners living within sight of the base say that Migs still take off and land at the base -- although "they are not many."

"They are hardly going out in droves," says one foreign resident with a military background, "because they don't have that many left." Around the front in southwestern Iran, near the threatened Khuzestan capital of Ahwaz, the Iraqi Air Force has been conspicuous by its absence.

During a week of visits along the whole front from the southern tip of the Shatt-al-Arab to the Iraqi front lines 15 miles south of Awaz, no planes were seen providing ground support to the exposed troops camped on the flat Khuzestan plain. Nor were they around to challenge the Iranian Phantoms that darted over the entrenched Iraqi forces picking off armored personnel carriers or supply trucks seemingly at will.

With their sophisticated SAM6 antiaircraft missile batteries kept back around Iraqi cities, mostly around Baghdad, and the main force of the remaining Iraqi Air Force planes also apparently being held for the defense of Baghdad, the Iraqi Army in the field has been left dangerously naked.

Field units seem equipped mostly with antiquated, manually operated antiaircraft guns of World War II vintage. The more luckly units in the field are equipped with radar-guided ZSU23s or self-propelled BSU57s, but they, too, seem inadequate for a fast-flying Phantom diving at 100 feet under normal radar screens.

As visitors to the front line can testify, the Iranian jets, which mostly fly singly, can swoop in over Iraqi positions, drop their bombs with surprising accuracy and be up in the air banking and flying home before the first antiaircraft batteries can even open up.

Fortunately for the Iraqis, the Iranian Air Force -- which totaled some 447 sophisticated U.S.-made fighters and bombers at the time the Shah of Iran was overthrown -- is also not fully operational.

Should the iranians manage to bring their remaining Air Force to bear on the four divisions of Iraqis who have penetrated Iran's western border in at least four points, it could prove to be a telling advantage in the war which, after more than one week, has no clear victor.