Iraqi forces pushed into one of the Islamic State’s most important strongholds Wednesday, breaking the militants’ hold on the city of Tikrit in what is shaping up to be a decisive battle for pro-government forces­ in their fight to eradicate the jihadists.

The gains by Iraqi troops and allied militia fighters could mark a crucial step in dislodging the Islamic State from other key areas, including the northern city of Mosul. But it was unclear whether Iraqi forces­ would hold their ground in Tikrit. Government troops have struggled to maintain control over recaptured territory in the past.

Iraqi television showed civilians in towns near Tikrit greeting pro-government forces­ as they swept through on their way to the city. Iraqi security officials said later Wednesday that the majority of Tikrit had been freed from Islamic State control but that battles continued in parts of the city. They said the jihadists had laced Tikrit with improvised explosive devices designed to slow the government advance. Much of the civilian population had already fled.

The military offensive includes a combined force of up to 30,000 troops, including a large contingent of Shiite militia fighters backed by Iran. Some officials and rights advocates have raised concerns about the sectarian nature of the operation and the possibility of retaliatory attacks against the local population, which is mostly Sunni.

Tikrit was the site of a massacre of as many as 1,700 Shiite soldiers by Islamic State militants in late spring. Shiite commanders have portrayed the current offensive as revenge for the slaughter, in which some local Sunni tribes also participated. The Islamic State, also known as ISIS or ISIL, has controlled the city since June.

In Washington, the Obama administration hailed the Tikrit operation as proof that its broader strategy in the region was working. White House press secretary Josh Earnest said that the Islamic State had been pushed out of 25 percent of the territory that its fighters took in the spring and summer in Iraq. He apparently was also referring to defeats suffered by the extremist Sunni combatants in Diyala province, just northeast of Baghdad, and in Kurdish areas near Mosul.

But the absence of U.S. warplanes in the Tikrit offensive appeared to expose rifts in the U.S.-Iraqi alliance. Iraqi officials have suggested that American air power was not necessary in the offensive, which began March 1.

U.S. planes have been striking Islamic State targets elsewhere in the country. The United States is also helping train Iraqi military units and has deployed about 300 advisers to al-Asad Air Base in the western province of Anbar.

Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, acknowledged Wednesday that Iranian-backed Shiite militia forces­ “dramatically” outnumbered Iraqi military and Sunni volunteer forces­ participating in the Tikrit operation. He said the pro-government side included about 1,000 Sunni tribal members, a brigade of 3,000 Iraqi troops, several hundred Iraqi military counter­terrorism forces­, and “approximately 20,000 of the popular mobilization force, which are the Shia militia.”

Dempsey, speaking in congressional testimony, said that Iranian participation in Iraq was “a positive thing in military terms against ISIL.” But, he said, “we are all concerned about what happens after the drums stop beating and ISIL is defeated, and whether the government of Iraq will remain on a path to provide an inclusive government for all of the groups within it. We’re very concerned about that.”

The Tikrit operation, he said, “will be a strategic inflection point one way or the other in terms of easing our concerns or increasing them.”

Another senior U.S. official expressed concern about the prominent role being played by Iran and its proxies, which he said was especially worrisome to the Persian Gulf Arab allies in the U.S.-led coalition battling the Islamic State.

“They are deeply concerned . . . that Iran is taking a stab at eliminating our contribution or minimizing it,” said the official, who spoke on the condition of anonymity to discuss sensitive diplomatic matters. “There is a concern over the struggle between the United States and Iran for strategic influence and lasting influence.”

90% ‘has been liberated’

Iraqi forces and militiamen entered Tikrit early Wednesday, approaching the city from all four directions, officials said. The fighters secured the largest neighborhood, Qadisiya, in the north, and moved on to clear the city’s industrial quarter, commanders said. Pro-government forces­ then seized the Tikrit Military Hospital. Iraqi helicopters were striking the militants as they gathered at key points in the city Wednesday night, according to officials.

Security forces­ continued to battle the militants Wednesday night on the edges of the vast presidential palace complex, which the jihadists have used as a base.

“Ninety percent of Tikrit has been liberated, but we are still fighting near the presidential palaces­ and in areas in the east,” said Emad al-Zahary, head of military operations in the nearby city of Samarra.

A spokesman for the Shiite militia Asaib Ahl al-Haq also said his men were engaged in clashes­ with the jihadists outside the complex Wednesday.

Tikrit, about 110 miles northwest of Baghdad, was the home town of former Iraqi strongman Saddam Hussein, and many of his former lieutenants joined the Islamic State. Hussein long favored his fellow Sunnis over the country’s Shiite majority. After a ­Shiite-led government came to power in the wake of the U.S.-led invasion in 2003, then-Prime Minister Nouri al-Maliki implemented sectarian policies that turned Sunni communities into breeding grounds of support for Islamic State militants.

It is likely that Iraqi forces­, even if they hold Tikrit, will face a prolonged insurgency in the area, analysts say. That will have implications for operations to retake Mosul, a city of about 1 million people still under the control of the Islamic State.

For months, government troops have been mired in seesaw battles with Sunni militants in areas of Anbar province, as well as the oil refinery town of Baiji, which is close to Tikrit. Iraqi forces­ seized Baiji in November but later ceded ground when the jihadists mounted a successful counter­offensive.

On Wednesday, the Islamic State targeted the Anbar provincial capital of Ramadi with multiple car bombs at key government installations, demonstrating the militants’ reach.

Iraqi security expert Safa al-Asam said that pro-government forces­ were not trained for urban warfare and ran the risk of getting bogged down in a guerrilla-style conflict with the militants in Tikrit.

If the Shiite militias also stay on to fight the militants, their presence could increase tension with local Sunni tribes.

“If they stay in Tikrit, there will be major problems,” Iraqi analyst Hisham al-Hashimi said. “I believe there will be assassinations and human rights violations. They will not leave land they liberated with their own blood.”

Greg Jaffe and Karen DeYoung in Washington and Mustafa Salim in Baghdad contributed to this report.

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