Baghdad, AP—Iraqi helicopter gunships struck suspected insurgent positions in Tikrit on Sunday as part of a government offensive to retake the northern city from Sunni militants led by the Al-Qaeda breakaway group, the Islamic State of Iraq and Syria (ISIS), residents and officials said.

The Iraqi military launched its push to wrest back Tikrit, the hometown of former dictator Saddam Hussein, on Saturday with a multi-pronged assault spearheaded by ground troops backed by tanks and helicopters. Security officials said the army is coordinating its campaign with the United States.

The predominantly Sunni city, a hotbed of antipathy toward Iraq’s Shi’ite-led government, is one of two major urban centers that fell to insurgents earlier this month during their lightning offensive across the country’s north and west.

The insurgents appeared to have repelled the military’s initial push for Tikrit, and remained in control of the city on Sunday, but clashes were taking place in the northern neighborhood of Al-Qadisiyah, two residents reached by telephone said.

Muhanad Saif Al-Din, who lives in the city center, said he could see smoke rising from Al-Qadisiyah, which borders the University of Tikrit, where troops brought by helicopter established a bridgehead two days ago. He said many of the militants in Tikrit had deployed to the city’s outskirts, apparently to blunt the military attack.

Military spokesman Qasim Al-Mousawi told reporters Sunday that the military was in full control of the university and had raised the Iraqi flag over the campus.

“The battle has several stages. The security forces have cleared most of the areas of the first stage and we have achieved results,” Mousawi said. “It is a matter of time before we declare the total clearing [of Tikrit].”

A provincial official reached by telephone confirmed that the insurgents retained control of most of Tikrit, and that the fighting was concentrated in Al-Qadisiyah. He also reported clashes northwest of the city around an air base that previously served as a US military facility known as Camp Speicher.

He spoke on condition of anonymity because he was not authorized to brief the media.

Jawad Al-Bolani, a security official in the provincial operation command, said the US was sharing intelligence with Iraq and has played an “essential” role in the Tikrit offensive.

“The Americans are with us and they are an important part in the success we are achieving in and around Tikrit,” Bolani told the Associated Press.

Washington has sent 180 of 300 American troops President Barack Obama has promised to help Iraqi forces. The US is also flying manned and unmanned aircraft on reconnaissance missions over Iraq.

Iraq’s government is eager to make progress in Tikrit after weeks of demoralizing defeats at the hands of ISIS and its Sunni allies. The militants’ surge across the vast Sunni-dominated areas that stretch from Baghdad north and west to the Syrian and Jordanian borders has thrown Iraq into its deepest crisis since US troops withdrew in December 2011.

More ominously, the insurgent blitz, which prompted Kurdish forces to assert long-held claims over disputed territory, has raised the prospect of Iraq being cleaved in three along sectarian and ethnic lines.

For embattled Prime Minister Nuri Al-Maliki, success in Tikrit could help restore a degree of faith in his ability to stem the militant tide. Maliki, a Shi’ite who has been widely accused of monopolizing power and alienating Iraq’s Sunni and Kurdish minorities, is under growing pressure to step aside. But he appears set on a third consecutive term as prime minister after his bloc won the most seats in April elections.

The government received a boost in its battle with the militants with the arrival in Baghdad late Saturday of five Sukhoi 25 warplanes purchased secondhand from Russia. The aircraft is designed to provide close air support to ground forces and to destroy mobile targets.

Iraqi air force commander Lt. Gen. Anwar Hama Amin said the military is “in urgent need of this type of aircraft during this difficult time.”

“These jets will enter service within a few days—the coming three or four days—in order to support the units and to fight the terrorist ISIS organization,” he said.

The planes could be deployed in the fight for Tikrit, a predominantly Sunni city with a population of more than 200,000, located some 80 miles (130 kilometers) north of Baghdad, where anger toward Iraq’s Shi’ite-led government runs deep.

ISIS, which already controls vast swaths of territory in northern and eastern Syria amid the chaos of that nation’s civil war, aims to erase the borders of the modern Middle East and impose its strict brand of Shari’a law.

In Iraq, the group has formed an alliance of sorts with fellow Islamist militants as well as former members of Saddam’s Ba’ath party, with all of them hoping to overthrow Maliki’s Shi’ite-led government.

The militants have tapped into deep-seated discontent among Iraq’s Sunnis, who largely dominated the country until the US-led overthrow of Saddam brought the Shi’ite majority to power. Since then, Sunnis have complained of discrimination and have said they are unfairly targeted by the country’s security forces.