BAGHDAD — Iraqi forces backed by Shiite militiamen and Sunni tribal fighters opened a large-scale operation Monday seeking to retake the Islamic State-held city of Tikrit, a critical stepping stone for wider attempts to reclaim territory in northern Iraq.

The offensive, announced on Iraqi state television, marks the third attempt by government forces to regain control of the city from the Islamic State militants, who seized it in June.

Tikrit, about 110 miles northwest of Baghdad and the birthplace of Saddam Hussein, is a strategic foothold along a major highway linking the capital and Mosul, the largest city in northern Iraq. Mosul is now held by the Islamic State.

Iraqi officials said more than 10,000 fighters – including soldiers, federal police, and Sunni and Shiite factions – attacked areas around Tikrit from three sides. They were backed by artillery fire and airstrikes from Iraqi fighter jets, officials said.

“This is only the first stage of the battle,” said Aday Thanoon, a council member in Salahuddin province, where Tikrit is located.

Thanoon and other officials said warplanes from a U.S.-led coalition that has been striking Islamic State targets in Iraq since August were not providing air cover for the Tikrit offensive.

“Maybe in the coming days there will be airstrikes from the coalition,” Thanoon said.

A spokesman for the coalition said it had not carried out any airstrikes because the Iraqi government did not request support.

Iraqi officials said troops “liberated” villages about 15 miles south of Tikrit on Monday afternoon. Pro-government forces encountered improvised explosive devices, snipers and booby-trapped houses on the way.

“The operation is not going to be easy. It will be hard,” said Jassim al-Jubara, the head of the area’s security committee.

As Jubara spoke by telephone on Monday, a man could be heard informing him that his 24-year-old nephew had been killed on the battlefield.

“Praise God,” Jubara responded. “We are all sacrificing for Iraq.”

Tikrit and surrounding areas are populated by Sunnis, who view the Shiite-led government in Baghdad with suspicion. The presence of Shiite forces – which have close ties to Shiite power Iran – in the offensive could be a test of Sunni cooperation in the region.

Hours before the operation, on Sunday night, Prime Minister Haider al-Abadi called on Sunni tribesmen fighting alongside the Islamic State to abandon the group.

In Tikrit last summer, Islamic State militants massacred as many as 1,700 Iraqi Shiite soldiers at Camp Speicher. Many Iraqi Shiites believe Sunni tribesmen helped kill the troops.

Rights groups, meanwhile, have accused Shiite militias of committing abuses against Sunni residents they suspect of aiding the militants.

Iran’s Fars News Agency said Monday that the offensive is being assisted by members of the Iranian Revolutionary Guard Corps, including the commander of the elite Quds Force, Maj. Gen. Qasem Soleimani.

Iran is a major opponent of the Islamic State, and Soleimani has been shown visiting various battle fronts in Iraq in recent months. But the latest report could not be independently confirmed.