The Iraqi soldiers are reportedly approaching the city from five directions, as they seek to prevent ISIS militants from either escaping from Tikrit or sending in reinforcements to bolster its defense.
The operation is part of a wide-scale offensive to retake Tikrit and Salahuddin province ordered by Iraqi Prime Minister Haider al-Abadi on Sunday.
It has highlighted the role played by neighboring Iran in the fight against ISIS, at a time when the United States and five other world powers are negotiating with Tehran on a controversial deal to curb its nuclear program
The semiofficial Iranian FARS news agency reports that Qassim Sulaimani, the commander of the elite Iranian Al-Quds Brigade, is helping oversee the operation to retake Tikrit.
has provided advisers, weapons and ammunition to the Iraqi government. According to the Pentagon, there may be Iranians operating heavy artillery and rocket launchers as well.
Despite the reported Iranian assistance, the Iraqi forces face no easy task as they seek to advance on Tikrit.
Besides the direct threat posed by ISIS
fighters, they must also avoid the large numbers of homemade bombs that litter the approaches to the city.
Iraq has not asked the U.S.-led coalition against ISIS to provide air cover for the operation to retake Tikrit.
ISIS released a number of propaganda images Wednesday, showing several vehicles and dozens of ISIS militants with their weapons, which it said were part of a military reinforcement of Salahuddin province.
CNN cannot independently confirm the authenticity of the photographs.
Long, hard slog may lie ahead
Tikrit fell to ISIS in June of 2014, after the group's capture of Iraq's second-largest city, Mosul. Tikrit is best known to Westerners as the birthplace of former Iraqi dictator Saddam Hussein
ISIS, the radical Sunni militant group, has been on a murderous campaign
to establish a caliphate across swaths of Iraq
On Monday, Iraqi forces approached Tikrit from several fronts
, Iraqiya TV reported, engaging with ISIS north of the city at al-Alam and south of the city at al-Dour.
The element of surprise probably was not a factor, as reports of Iraqi troops amassing near Tikrit were widely shared.
What awaits the Iraqi army is most likely a long, hard slog and not a quick rout. Tikrit is a big city, and the army and its associated militias have had problems recapturing much smaller towns from ISIS.
There have been several failed attempts to recapture Tikrit since the second half of 2014. While Iraqi forces have gained some territory in the area, it has generally been under ISIS control for the last eight months or so.
Prime Minister al-Abadi, who is also commander in chief of the armed forces, said on Twitter that he would "oversee the operation to liberate Tikrit" from ISIS.
Shia and Sunnis fight together
The joint Iraqi forces fighting to retake Tikrit include Iraqi troops, members of the Shia al-Hashed al-Shaabi militia, members of the Sunni Sons of Salahuddin brigades and other Sunni tribal fighters.
The offensive involves around 30,000 fighters in all.
Rights group Human Rights Watch on Wednesday urged government forces to protect civilians from revenge attacks by pro-government militias as they fight to retake Tikrit.
The rights group said it had documented "numerous atrocities against Sunni civilians by pro-government militias and security forces" after they recaptured other towns.
Human Rights Watch also warned that ISIS could seek to use civilians as human shields.
"All commanders in Tikrit need to make sure that their forces protect civilians and allow them to flee the combat zone," said Joe Stork, the group's deputy Middle East and North Africa director.
"Past fighting raises grave concerns that Tikrit's civilians are at serious risk from both ISIS and government forces, and both sides need to protect civilians from more sectarian slaughter."