Western diplomats here have warned their countries that economic sanctions against Iran would cripple political efforts that they believe are under way within the government to gain the release of the American hostages held for the past nine weeks at the U.S. Embassy.

Meanwhile, factional fighting continued to spread throughout Iran today and three people were reported killed in northwest Tabriz during the fourth day of fighting between rival militiamen. More than 150 people have been reported injured since Friday in the clashes between followers of the nation's paramount spiritual leader, Ayatollah Ruhollah Khomeini, and those of rival Ayatollah Kazem Shariatmadari, chief mullah of the province of Azerbaijan.

Tens of thousands of people marched through the streets of Tabriz, the center of Iran's large Turkish-speaking Azerbaijani minority, setting fire to the headquarters of a pro-Khomeini committee and ripping down Khomeini posters, according to the state radio.

Religious and ethnic violence also flared along the southern Persian Gulf coast, in south east Sistan-Baluchistan Province and in the western Kurdish areas of the country.

As the United Nations Security Council returns today to the question of sanctions, diplomatic observers here said any attempt to punish Iran would strengthen the hand of the Islamic militants who refuse to release the U.S. hostages until deposed Shah Mohammad Reza Pahlavi is returned to Iran.

In recent weeks officials in the Foreign Ministry and the rulling Revolutionary Council have tried to persuade Khomeini to urge the students to free the hostages and focus on the job of rebuilding the nation.

But approval of a U.S. proposal for economic reprisals, according to diplomatic sources, would be seen by Iranian leaders as undue provocation and discredit any internal appeal for flexibility on the hostage issue.

Khomeini, the nation's political as well as spiritual leader is in no position to be provoked, diplomats say, because of the growing domestic pressure caused by regional conflict.

The latest twist in Iran's internal debate has been the demand by the embassy captors that the Foreign Ministry hand over from its custody U.S. Charge d'Affaires Bruce Laingen for questioning about alleged espionage documents.

Foreign Minister Sadegh Ghotbzadeh, who has rebuffed past demands that Laingen be turned over, referred the decision to Khomeini two days ago.

If Khomeini remains silent or refuses to deliver Laingen for questioning, diplomatic observers believe it would be a victory for moderates who see Laingen as a liaison in negotiations with Washington.

But a directive for the Foreign Ministry to hand over Laingen would be interpreted as a damaging blow to these moderates and an indication that Iran's spiritual leader continues to throw his weight behind the Islamic militants in the embassy.

Khomeini has not indicated if or when he will make a decision. A spokesman at his headquarters in the holy city of Qom said today the 79-year-old cleric plans to take a two-week vacation beginning Jan. 21.

As regional unrest spreads around him, Khomeini has shown less and less interest in the hostage issue, rendering no known public statements on the hostages in days and refusing to meet U.N. Secretary General Kurt Waldheim during his mediation mission here last week.

Although provincial insurgents have been making demands for months, the recent trouble in Tabriz is expected to be especially alarming to Khomeini, because the Turkish-speaking population centered in that city is the largest ethnic minority in the nation.

Not only does the mounting violence in Iran's provinces challenge Khomeini's authority, but also it is believed to distract him from the hostage issue and the brewing debate with Iran between moderates and hardliners on how the crisis should be resolved.

Azerbaijan's Shariatmadari is generally considered the second most influential cleric in Iran. His followers and those of Khomeini have been in conflict since Shaariatmadari urged a boycott of the recent voting for a constitution that conferred supreme authority on Khomeini.

In other regions, too, local resentment at the concentration of power in the hands of central authorities has boiled over into clashes with Khomeini's Revolutionary Guards.

In addition to the latest Tabriz fighting and demonstrations, another killing was reported in the southern Persian Gulf coastal town of Bandar Lingeh, bringing the death toll in fighting between rival Islamic groups to 42 since Saturday. More than 100 people have been injured in the clashes between adherents of the Sunni and Shiite (the majority faith in Iran) wings of Islam.

Similar Sunni-Shiite tensions in southeast Sistan-Baluchistan claimed eight more lives last night as the demoninant Sunni tribesmen continued their fight to remove Shiite central government representatives from their province. f

In the Kurdish province of western Iran where insurgents held off government forces last year, Governor General Hossein Shahvessi resigned his post to join Kurdish rebels demanding the removal of central government security forces from the provincial capital of Sanandaj.

Meanwhile, less than 12 hours after the Iranian government threatened the mass expulsion of American, British and West German reporters, revolutionary militiamen in northwest Tabriz ordered foreign newsmen, including a CBS-TV crew, to leave the city immediately.

CBS reporter Doug Tunnell said two members of the paramilitary outfit arrived at the International Hotel at breakfast with an expulsion order signed by the head of the provincial revolutionary court. The guards escorted Tunnell and his cameramen to the airport.