Security forces backed by helicopters battled supporters of a radical cleric in the Shiite holy city of Karbala on Wednesday, as spreading violence threatened to pull more areas of the country into turmoil.

The clashes erupted when the security forces tried to seize the offices of Shiite cleric Mahmoud al-Sarkhi, who has sharply criticized the government. The fighting marked the first sign of a potential for violent rifts within the Shiite community as the government battles a Sunni insurgency inspired by al-Qaeda. Two members of the security forces were killed, along with an unconfirmed number of the cleric’s gunmen, according to a local official.

Prime Minister Nouri al-Maliki is trying to prevent the breakup of Iraq in the face of an offensive by the heavily armed insurgents, who have already declared an Islamic state stretching across Iraqi and Syrian territory. But the threats to Iraq’s territorial integrity are many, as the Kurds prepare to vote on independence farther north and Shiite dissatisfaction bubbles in the south.

With violence engulfing the country, Maliki on Wednesday offered an amnesty to Sunni tribesmen who have joined the insurgency, his latest attempt to claw back control. In his weekly televised address, the embattled prime minister called on tribal leaders to stand behind the Iraqi state, although he said that in cases of “spilled blood” it would be up to victims’ families to decide whether the tribesmen should be forgiven. After overrunning the northern city of Mosul on June 10, Sunni fighters of the Islamic State of Iraq and Syria (ISIS), now renamed Islamic State, have seized territory in the north and west and proclaimed a caliphate on captured lands.

“They should return to their senses,” Maliki said of the tribesmen. “I welcome them. I welcome them back. I welcome their unity with their brothers from other tribes.”

The United States successfully brought Sunni tribesmen on board to battle al-Qaeda as part of the Sunni Awakening movement starting in 2005. But Maliki, a Shiite, has not kept up payments or fulfilled promises to incorporate the Sunnis into the security forces, stirring resentment.

He has also faced opposition from Shiites themselves, including Sarkhi in Karbala, about 55 miles southwest of Baghdad.

Iraqi forces attempted to enter Sarkhi’s compound just after midnight, and his armed supporters battled them for nine hours, said the local official, who spoke on the condition of anonymity because he was not authorized to provide information. He said helicopters fired on the compound.

It was not immediately possible to confirm the death toll, which Iraqi news media put at between three and 14. Sarkhi, who split from the anti-American Shiite cleric Moqtada al-Sadr in 2004, posted pictures of slain supporters on his official Web site. Another photo showed what was described as a burning Iraqi military Humvee.

“These militia-like actions are a result of the stand of [Sarkhi], who rejects division and sectarianism, which has killed the people of Iraq,” said a statement on the site.

After the clashes, Karbala, home to one of Shiite Islam’s most revered shrines, was on lockdown Wednesday, with cars banned from the roads as security forces attempted to clear the area. Sarkhi has expressed opposition to Iraq’s leading Shiite cleric, Grand Ayatollah Ali Sistani, who has called on all able-bodied Iraqis to take up arms against the radical Sunni insurgents. He has also repeatedly tried to take control of the city’s Imam Abbas shrine, the local official said.

In a sign that the unrest could spread, supporters of the cleric attempted to attack a police station in Diwaniyah, about 70 miles southeast of Karbala, according to residents. In the southern oil port of Basra, meanwhile, protesters blocked roads with burning tires, although the provincial governor told television stations that the action was unrelated to Karbala’s violence.

Maliki stressed Wednesday that Iraqi civilians should take up arms only under the authority of the security forces. He also rejected calls for independence by the semiautonomous Kurdish region, saying that such a move would be unconstitutional.

“Firstly, I’d like warn the Kurdish people, who are treated unjustly, that this will harm you,” he said. “It will throw this region in a labyrinth that you will not be able to escape. Secondly, you decided, according to the constitution, to be part of Iraq.”

He criticized the Kurds, who have been expanding their areas of control into disputed territory, for seizing the oil-rich northern city of Kirkuk without a referendum.

His speech came just a day after the first session of Iraq’s newly elected parliament broke up in disarray, with no progress on forming a new government. The legislative session ended in heated arguments and a walkout, an indication of the divisions besetting the country.

Maliki, who is trying to secure a third term in office despite dwindling support, said he hoped that next week’s parliamentary session would be more productive and that factions would be “realistic.” While the political process is important, he said, there must also be a focus on the “battle.”

Amid the violence, Maliki has been trying to bolster Iraq’s air power, with neighboring Iran stepping in as Baghdad waits for U.S. deliveries of F-16 fighters and Apache helicopters. Iran has returned several Soviet-era SU-25 fighter jets to Iraq, according to the International Institute for Strategic Studies, which analyzed serial numbers in images released by the Iraqi Defense Ministry this week.

The jets were abandoned in Iran by fleeing Iraqi air force pilots during the Persian Gulf War in 1991 and Iran impounded them, claiming the planes as war reparations. Iraq has been urgently requesting their release since the violence broke out.

Ahmed Ramadan in Beirut contributed to this report.