Iraqi Shiite Muslim rebels Monday reported fierce fighting with pro-Saddam units in a besieged holy city, while Kurdish guerrillas claimed further gains. Both reported large-scale troop defections.

Shiite insurgents, insisting on anonymity, said shelling by tanks of Saddam Hussein's loyal Republican Guard has killed or wounded 500 people in the holy city of Karbala in southern Iraq. They said the rebels have blockaded themselves in shrines inside the city.But official sources in Baghdad said the troops re-established control of Karbala Monday, but they gave no details.

Kurdish guerrillas battling in the mountainous north claimed that the oil center of Kirkuk will soon fall and reported they have seized more territory after 10 days of fighting.

Near Baghdad, Iraq's capital, about 8,000 pro-Saddam troops and ruling Baath Party militiamen have sealed off large sections of suburban Saddam City held by rebels, opposition sources reported.

Travelers fleeing Iraq said that the troops, including Republican Guard units, were shelling the al-Thawra neighborhood there and that helicopter gunships were strafing the streets and buildings.

The travelers said al-Thawra and other predominantly Shiite quarters have been under a curfew since anti-Saddam protests broke out last week.

The violence flared when the bodies of hundreds of soldiers, killed in Iraq's six-week war for Kuwait, were returned to their families for burial. The state-run media had proclaimed that Iraq had won the war.

In Beirut, Lebanon Monday, representatives of 23 opposition groups - including Communists, Arab nationalists and Shiite Muslim fundamentalists urged Pope John Paul II and other religious leaders to demand that Saddam Hussein's forces stop "massacring" rebels.

Shiite Muslim leader Ayatollah Taqi al-Mudarressi, speaking to reporters as the Iraqi rebels gathered at the heavily guarded Bristol Hotel in west Beirut, said he had asked for foreign religious intervention to help stop the troops.

"I have sent letters to the world's religious leaders, including the pope, to prevent prevent Saddam's criminal regime from massacring the Iraqi people," he said, adding that there is a "strong possibility that Saddam would use chemical weapons to quell the uprising in Karbala and other parts of Iraq."

Saddam used chemical weapons against a Kurdish uprising in northern Iraq in 1988. The United States has said his chemical capability is believed to have been largely destroyed in the war.

Al-Mudarressi, spiritual leader of the Shiite Islamic Labor Organization, said that because Karbala is a holy city, "it is the responsibility of the world's religious leaders to protect it."

Karbala, 60 miles south of Baghdad, is revered by Shiites as the burial place of Imam Hussein, grandson of the Prophet Mohammed, Islam's founder.