Rockets and mortars have echoed across the Iraqi city of Tikrit as the country's security forces continue to engage fighters of the Islamic State of Iraq and the Levant (ISIL), a day after forcing their way into the city.

The offensive to recapture Tikrit is seen as a key step towards dislodging the group, which seized much of northern and western Iraq last summer and controls about a third of Iraq and Syria.

 Al Jazeera's Imran Khan analyses the battle

Iraqi troops and allied Shia militiamen entered Tikrit for the first time on Wednesday from the north and south.

On Thursday, the Iraqi Defence Minister visited the troops in Tikrit, the birthplace of Saddam Hussein, who was ousted as president in 2003 and executed three years later.

"Our visit is aimed to support our forces and congratulate them on the victories they achieved," Khalid al-Obeidi said.

Lieutenant-General Abdul-Wahab al-Saadi, head of military operations in Salahuddin, said "the military operation is still going on till the liberation of all Salahuddin".

Tikrit, the capital of Salahuddin province, sits on the Tigris River about 130km north of Baghdad.

Several of Saddam's palaces remain there, and supporters of the deceased president are believed to have played a key role in ISIL's seizure of the city last year.  

Homes burnt

The United States, meanwhile, has expressed concern over reports that Shia militia set fire to homes as they advanced on Tikrit, but officials did not confirm cases of abuse during the major offensive against ISIL.

US officials said they were closely monitoring accounts from Tikrit, including video posted on social media, showing buildings being set ablaze. But attributing blame was tricky, they noted, pointing to accusations against both the Iran-backed militia and ISIL fighters.

"What we know is that there are houses on fire," said one US official, speaking on condition of anonymity.

The official said civilian casualties were believed to be very limited, since many local residents had long fled the city.

ISIL fighters stormed into Tikrit last June during a lightning offensive that was halted just outside Baghdad. They have since used the complex of palaces built in Tikrit under Saddam Hussein as their headquarters.

The US says Baghdad did not seek aerial backup from the US-led coalition in the Tikrit campaign. Instead, support on the ground has come from neighbouring Iran, Washington's long-time rival in the region. Tehran has sent an elite Revolutionary Guard commander to oversee part of the battle.

Source: Al Jazeera and agencies