A significant number of Islamic State group fighters have withdrawn from northern Iraqi cities, officials in Tikrit and Mosul say, as government forces prepare for another major ground and air offensive.

Sources told Al Jazeera that security forces had entered the outskirts of Tikrit from three directions, including Tikrit University and the provincial government building.

Reports said that a division of the Iraqi army had made advances backed by air, mortar and artillery fires targeting several positions across Tikrit, which has been under Islamic State control for months.

It was not clear if the air strikes were carried out by US or Iraqi jets, however US aircraft have been dropping leaflets warning residents of the impending bombing of Islamic State sites.

At least two local brigades from Tikrit have also reportedly joined the Iraqi forces fighting Islamic State fighters.

Rashid al-Bayati, a member of the provincial council of Salahuddin that includes Tikrit, told local media on Tuesday that there has been “major military presence” on the outskirts of Tikrit, and that he expected an imminent ground offensive.

Sources told Al Jazeera that one town leading to Tikrit, al-Alam, had come “under wrong bombardment” by Iraqi warplanes, killing “dozens” of displaced civilians overnight, Al Jazeera cannot independently confirm the report.

In Kirkuk, the Iraqi army announced that 400 Islamic State fighters have fled, and that security forces have seized explosives, weapons and munitions, security officials said.

Khalid al-Mifriji, a parliament member for Kirkuk, was quoted as saying that 14 people, including women and children, were killed or injured during an air strike on Daqouq town, south of the city.

Al Jazeera’s Jane Arraf, reporting from Baghdad, cautioned that it would be “incredibly difficult” for the government forces to drive out the Sunni armed group in cities that are also dominated by Sunnis.

“I would not claim victory quite so fast, if I were the Iraqi government. They are simply not ready,” Arraf said, adding that in Sunni areas, “Sunnis have to be in the lead” against the Islamic fighters.

Iraq’s defence ministry said that Faris Abdo al-Afri, an Islamic State leader, and several of his aides were seriously injured during air strikes in northwestern Mosul.

The Islamic State has reportedly been destroying buildings in the area, including blowing up the Imam Abdul-Rahman shrine in the al-Tawalib district of central Mosul.

In Anbar province, there are reports that 2,000 government troops and pro-government fighters are massing to carry special missions against the Sunni rebels, while tribal fighters are aiding the battle in western Iraqi cities, including Fallujah.

‘Ghost town’

Earlier, government forces led by Kurdish fighters and backed by Shia armed volunteers regained control of the towns of Sulaiman Bek and Amerli, driving out the rebel Sunni fighters and liberating the towns under siege for weeks.

Al Jazeera’s Sue Turton, who reached Sulaiman Bek, described it as a “ghost town”.

However, as aid begin to trickle in, people are worried that many Islamic State fighters may have “blended back into the surrounding communities,” Turton said.

Earlier, she also reported about the celebration in Amerli, a town of mostly Shia Turkmen.

In Tuz Kharmatu, the Iraqi army has taken control of several Sunni areas, while some other villages remain under the control of tribal leaders, with the Islamic State controlling some pockets of the town.

The Islamic State has declared a “caliphate” in regions under its control in Iraq and Syria, and has engaged in mass killings as it swept through swathes of territories north of Baghdad in June. Al Jazeera