Iran-backed Iraqi army and Shiite militias entered the city of Tikrit on Wednesday, authorities said, breaching one of the biggest strongholds of Islamic State of Iraq and al-Sham (ISIS) in a key test for Iraqi forces. Allied Iraqi forces entered the city through its northern Qadisiyya neighborhood, where authorities quickly established a supply line to reinforce troops, Salahuddin police Brig. Kheyon Rasheed told the state-run Iraqiyya television. "The terrorists are seizing the cars of civilians trying to leave the city and they are trying to make a getaway," Rasheed said. It wasn't immediately clear whether the troops and militiamen faced any resistance. A local official in Iraq's Salahuddin province also confirmed that Iraqi troops and the militias made it into Qadisiyya. He spoke on condition of anonymity as he was not authorized to brief journalists.

ISIS holds about a third of Iraq and neighboring Syria in its self-declared caliphate. Tikrit, the capital of Salahuddin province about 130 kilometers north of Baghdad, is one of the largest cities held by ISIS militants and lies on the road connecting Baghdad to Mosul. Retaking it will give Iraqi forces a major supply link to retake Mosul. On Tuesday, Iraqi forces retook the town of Alam on the outskirts of Tikrit. They also sealed off Tikrit to prepare for an offensive inside the city. Hidden bombs and snipers had slowed the troops' progress.

Iranian military advisers have been helping guide Iraqi forces in their advance on Tikrit. Among those directing operations is Iranian Gen. Qassem Soleimani, commander of the powerful Revolutionary Guard's Quds Force. The overt Iranian role and the prominence of Shiite militias in the campaign have raised fears of possible sectarian cleansing should Tikrit, an overwhelmingly Sunni city, fall to the government troops. The overt Iranian role and the prominence of Shiite militias in the campaign have raised fears of possible sectarian cleansing should Tikrit, an overwhelmingly Sunni city, fall to the government troops. The U.S. and Human Rights Watch (HRW) warned an offensive to retake control of the Iraqi city of Tikrit must not fuel sectarian tensions. In a report released by HRW last month, it was revealed that Iraqi people are not only suffering from atrocities committed by ISIS militants but also from armed government-backed Shiite militants. The report said the militias work closely with Iraqi government forces. "Abuses by militias allied with Iraqi security forces in Sunni areas have escalated in recent months. Residents have been forced from their homes, kidnapped, and in some cases summarily executed."

The U.S. says its allied coalition carrying out airstrikes targeting the militants has not been involved in the ongoing Tikrit offensive. Iraq's Prime Minister Haider al-Abadi has appealed for more aid for the country's beleaguered ground forces, which have yet to score a decisive victory against ISIS despite seven months of U.S.-led coalition air raids.