Pakistanis try to sort out responsibility for spate of bomb blasts. Any of several foreign or internal feuds could be behind sabotage

In the wake of the bomb blasts which killed at least 72 people in Karachi Tuesday, Pakistanis are questioning who might have been behind the attacks. The government of President Mohammed Zia ul-Haq has been unable to catch saboteurs responsible for earlier bomb blasts, which have killed some 150 people so far this year, or to pin the blame firmly on any particular country or faction.

Several foreign quarrels that have spilled over onto Pakistani soil - such as the Afghan and Iranian conflicts - could be responsible. So could one of many internal feuds dividing this Muslim society, observers say. Angry bystanders questioned at the scene of yesterday's carnage were at a loss to say who was behind it.

Some speculated that the blasts might be linked to the impending trial of five Palestinians accused of killing 22 people and wounding more than 100 in the hijack of an American airliner in Karachi last September.

Other bystanders pointed at the long-standing rivalry between Shiite and Sunni Muslim sects, as well as at ethnic feuds that have erupted in Karachi in recent months.

The provincial government of Sind, which includes Karachi, pointed immediately to outside involvement with a press statement that said, ``The explosions appeared to be the work of saboteurs of foreign origin.''

Government officials have in the past favored the Afghan explanation. In recent years, bomb explosions have been relatively frequent in the Northwest Frontier Province (NWFP), which borders Afghanistan. Many of those attacks have been aimed at Afghan resistance groups that have offices in Pehsawar or at some of the more than 3 million Afghan refugees in Pakistan.

Pakistani officials have accused the Soviet-supported communist Kabul government of sending saboteurs to destabilize the province and pressure Pakistan to clamp down on mujahideen (guerrilla) activities. Kabul denies the charges.

The NWFP is ideal territory for sabotage operations: It is awash with arms, many of them provided by the guerrillas' Western and Arab supporters, and torn by tribal and ethnic feuds, both among the Afghan groups and among the Pathan mountain tribes.

Western diplomats in Islamabad say the expansion of the bombing to other parts of Pakistan since last March, especially to the traditional heartland of Punjab Province, poses General Zia's government with a serious political problem.

One attack killed several leaders of a militant orthodox Sunni group on March 23 in Lahore. The group was known for being opposed to the brand of Islamization pursued by Zia during his 10 years in power, and also for hostility to the minority Shiites. Other Lahore bombings last week came on the eve of a major Shiite rally, at which speakers demanded a greater political say for their community. Tuesday's car bombings in Karachi occured near a mosque belonging to a predominantly Shiite subsect, the Bohris.

The main grounds for suspecting an Iranian angle relate to a violent clash in two cities last week. Three people died and more than 20 were hurt when gunmen attacked houses occupied by an exile Iranian opposition group. Several dozen Iranians were rounded up. Pakistan warned it would not tolerate anti-Khomeini activities on its soil, stressing that relations with Tehran were excellent.