The Iraqi government launched its biggest push yet to wrest back ground lost to Sunni militants, as soldiers backed by tanks and helicopter gunships began an offensive today to retake the northern city of Tikrit.

There were conflicting reports as to just how much headway the Iraqi military made in its initial thrust toward Tikrit, the hometown of Saddam Hussein.

Residents said militants were still in control of the city by nightfall, while Iraqi officials said the troops had reached the outskirts and even entered Tikrit itself.

What was clear, however, was the government's desire to portray the campaign as a significant step forward after two weeks of demoralising defeats at the hands of insurgents led by the al-Qaeda breakaway Islamic State of Iraq and the Levant.

The militants' surge across much of northern and western Iraq has thrown the country into its deepest crisis since US troops withdrew in December 2011, and threatens to cleave the country in three along sectarian and ethnic lines.

Iraq's large, US-trained and equipped military melted away in the face of the militant onslaught, sapping morale and public confidence in its ability to stem the tide, let alone claw back lost turf.

The Tikrit operation, if successful, could help restore a degree of faith in the security forces, as well as embattled Prime Minister Nouri al-Maliki.

Today's fighting began before dawn with helicopter gunships carrying out airstrikes on insurgents who were attacking troops at a university campus on Tikrit's northern outskirts, Iraqi military spokesman Lt General Qassim al-Moussawi said.

The government forces had established a bridgehead on the university's sprawling grounds after being airlifted in the previous day.

Sporadic clashes continued throughout the day at the university. At the same time, several columns of troops pushed north toward Tikrit from Samarra, a city along the banks of the Tigris River and home to an important Shiite shrine, a senior security official said on condition of anonymity because he was not authorised to brief the media.

By sundown, Lt General Ahmed Abu Ragheef, a commander in the Salahuddin Operational Command, said a column of troops had reached the edge of Tikrit, while another had secured an air base that previously served as a US military facility known as Camp Speicher.

The governor of Salahuddin province, Ahmed Abdullah al-Jabouri, told The Associated Press that troops pushed into Tikrit itself, reaching as far the provincial council building. 

However, Tikrit residents reached by telephone today evening said militants were still in control of the city and patrolling the streets.

They confirmed the clashes around the university, and reported fighting between the Islamic State and Iraqi forces to the southeast of the city as well.

Some residents described black smoke rising from a presidential palace complex located along the edge of the Tigris River after army helicopters opened fire on the compound.

They spoke on condition of anonymity out of concern for their safety.

Many locals had already fled the city in anticipation of a government assault, said another Tikrit resident, Muhanad Saif al-Din.

"Tikrit has become a ghost town because a lot of people left over the past 72 hours, fearing random aerial bombardment and possible clashes as the army advances toward the city," Saif al-Din said.

"The few people who remain are afraid of possible revenge acts by Shiite militiamen who are accompanying the army. We are peaceful civilians and we do not want to be victims of this struggle."

He said the city has been without power or water since last night.

The military also carried out three airstrikes on the insurgent-held city of Mosul early today. One of the air raids hit a commercial area that did not have an obvious military target, residents said.

South of Baghdad, heavy clashes between security forces and Sunni insurgents in the town of Jurf al-Sakhar killed at least 21 troops and dozens of militants, police and hospital officials said on condition of anonymity because they were authorized to brief the media.

Jurf al-Sakhar, located some 50 kilometres outside the capital, is part of a predominantly Sunni ribbon that runs just south of Baghdad.

The Islamic State, which has been joined by fellow Islamic militants as well as former members of Saddam's Baath party, has seized upon deep-seated discontent among Iraq's Sunni community with al-Maliki's Shiite-dominated government.

Al-Maliki has been widely accused of monopolising power and alienating Sunnis, and his failure to promote national reconciliation has been blamed for fueling Sunni anger.

The United States and other world powers have pressed al-Maliki to reach out to the country's Sunni and Kurdish minorities and have called for a more inclusive government that can address longstanding grievances.