Pro-government forces give the victory sign as they advance in Salahuddin province, where Tikrit is located, on March 2, 2015. The Iraqi government has launched an offensive to retake the city . (Thaier Al-Sudani/Reuters)

Iraqi forces face a prolonged fight to reclaim the northern city of Tikrit from the Islamic State, military experts and analysts said, as pro-government forces continued a large-scale offensive to oust the militant group from one of its major urban strongholds.

The militants on Tuesday carried out suicide attacks and detonated roadside bombs on the outskirts of the city, about 110 miles northwest of Baghdad, to slow the Iraqi troops’ advance. The Tikrit offensive, which started Monday, is the Iraqi government’s most ambitious military operation yet to uproot the Islamic State from territory the group seized last summer. Securing the majority-Sunni city is crucial for Iraqi forces to prepare for an offensive to retake nearby Mosul, which is also in the hands of the Islamic State.

[Watch: Islamic State smashes artifacts in Mosul]

But the attacks against government forces Tuesday signaled that the battle could morph into an urban war of attrition, with the potential for significant casualties. And even if the pro-government force of tens of thousands of soldiers and allied militia fighters sweeps in to rout the militants, Iraqi authorities are likely to encounter a long-term insurgency by supporters of the Islamic State, analysts say.

“In Tikrit, it will be a war inside the city, street to street,” said Safa al-Asam, an Iraqi security expert. “Our army has not trained for this kind of war, so there might be a lot of civilian casualties.” Most of Tikrit’s civilian population has fled, officials said.

Tikrit fell to Islamic State fighters in June during their blitz across northern Iraq. In the wake of the onslaught, the militants kidnapped and massacred as many as 1,700 Iraqi soldiers — most of whom were Shiites — from Camp Speicher, a former U.S. military base outside Tikrit. The militants hunkered down in the city, the home town of Saddam Hussein. Loyalists of the late strongman in Tikrit allied with the militants to challenge the Shiite-led government in Baghdad, solidifying control.

Shiite militia leaders have painted the new offensive as revenge for the Speicher massacre, raising fears of attacks on Sunni residents.

“We are very concerned the militias will commit violations against civilians when they get into the city,” said Abd Thiab al-Ajeeli, a Sunni lawmaker from Salahuddin province, where Tikrit is located.

The militias “accuse the tribes of supporting the Islamic State, and the people are afraid. They are afraid the militias will take revenge against them,” he said.

[Map: The Islamic State’s massacres in Syria and Iraq]

On Tuesday, pro-government forces began encircling Tikrit in a bid to isolate Islamic State fighters and cut off their supply lines. The siege, officials said, eventually will exhaust the militants.

Local officials, however, said the Islamic State has lined the city with explosives and deployed snipers to take out invading troops.

Some of the Shiite militia members have combat experience in Syria, where they allied with the army against the insurgency there. But most of the volunteer fighters — known as the popular mobilization forces — are ill-equipped to capture and hold a major urban center. In the past, Iraq’s government has struggled to maintain control of areas recaptured from the Islamic State.

Security forces say they plan to dispatch thousands of Sunni fighters and police officers to hold the city after the Islamic State is gone. Also, officials say, the presence of Sunni tribesmen will deter Shiite reprisal attacks and will free up the Shiite militia members to fight elsewhere.

But Sunni tribes battling the Islamic State say the government has not supplied them with enough weapons for the fight. In the district of al-Alam near Tikrit, the Jubouri tribe staged an uprising against the extremists last summer but quickly ran out of bullets and was forced to acquiesce. Analysts said that if the Sunni population feels marginalized by the government, the militants could again make inroads.

Iraq’s army relied heavily on U.S. troops to root out the urban insurgency that flowered in the wake of the 2003 American-led invasion.

U.S. forces persuaded thousands of Sunni tribesmen to battle fighters for the insurgent group al-Qaeda in Iraq — the predecessors of the Islamic State forces — in 2006 and 2007, but the Shiite Iraqi authorities in Baghdad later dismantled the initiative and sidelined Sunnis from positions of power. A Sunni resistance movement against the government later paved the way for the Islamic State’s sweep of key Sunni cities across Iraq.

Sayed al-Jayashi, an Iraqi military analyst, predicted that the government eventually will triumph in Tikrit.

“The Sunni tribes will be responsible for holding Tikrit — not the army or the popular mobilization [forces] — and they will avoid the mistakes the militia fighters made” against Sunnis in other areas, he said.

“Then the government will rebuild [Tikrit], restore electricity and allow the Sunnis to go back,” he said, referring to Sunni civilians. “It will take a long time, but eventually it will all be finished.”

Mustafa Salim contributed to this report.