The latest in a series of oil pipeline explosions today raised new fears of sabotage in Iran's vital oil-producing region.

The cause of the blast, which apparentlly occured at midnight, has not been established. But it has spurred new efforts to tighten security in the area where weekend explosions damaged as many as nine pipelines, most of them carrying oil products from the world's largest refinery at Abadan to the export terminal of Bandar Mahshahr.

Shortly after the explosion was reported, the head of the National Iranian Oil Company, Hassan Nazih, contacted military commanders to discuss increased security for oil installations in the southeastern province of Khuzestan.

Among those contacted was the military police chief, General Amir Rahimi, who was reinstated by Moslem leader Ayatollah Ruhollah Khomeini yesterday after being fired by the government. Rahimi announced that he has 3,000 men ready for deployment in Khuzestan.

The effect of the latest explosion, which damaged a pipeline carrying crude oil from Ahwaz at the center of the oil fields, to Abadan refinery, was not believed to be serious.

Production at the refinery has already been reduced to 100,000 barrels a day, or a fifth of capacity, following the weekend explosions.

The full extent of the damage from the earlier blasts is still not known. Nazih said today that repairs should be completed in five or six days, contradicting earlier estimates that only three or four days were needed for the job.

The authorities' main concern is to guard against sabotage of installations producing and exporting crude oil, in the area where Iranian Arabs have been agitating with increasing vigor for local autonomy.

There appears to have been some apprehension among Arab leaders about the news eof Rahimi's readiness to intervene in the province. In recent statements, Rahimi has taken a tough line on the demands of ethnic minorities seeking autonomy, particularly those of Iranian Arabs.

However, Rahimi said he would not be moving forces into the province without orders to do so from the government, which so far does not appear to be preparing for any major troop buildup.

Defense Minister Gen. Taqi Riahi commented that there were already enough troops in the province and that there was "no need for reinforcement for the time being."

Other incidents in the area underline the continuing threat to the stability of the province, where substantial quantities of arms are believed to have been stored by the Arab community.

Two men were killed during a shootout that started when Islamic Revolutionary Guards opened fire on a boat near Minoo island, located in the narrow Shatt-al-Arab waterway dividing Iran and Iraq.

The governor of nearby Khorramshahr, Mohammed Alavi, said the boat, which was captured, was found to contain Soviet-made RPG7 bazookas. One of the boat's crew, wounded and captured in the shooting, was an Iraqi, the governor said, adding that he was now under interrogation.

In Khorramshahr, a grenade attack on a railway club was reported. In addition, railway tracks close to the town were damaged in an explosion.

In another Iranian development, revolutionary courts in three cities executed three woman and three men on charges ranging from moral crimes to sabotage and murder of anti-shah demonstrators, the state radio reported early Thursday.

[The three women, all accused of prostitution-related crimes in Tehran, were the first women executed since the February revolution. The latest executions brought to 326 the number of deaths sentences carried out since the revolution.]

[The radio said the three women were found guilty of "purchase and sales of girls and promotion of obscenity."]