BAGHDAD — Iraqi soldiers and Shiite militiamen entered the Islamic State-held city of Tikrit on Wednesday, an official and a witness said, a key test for Iraqi forces in their battle against the Islamic State militants.

The local official in Iraq’s Salaheddin province said allied Iraqi forces entered Tikrit through its northern Qadisiyya neighborhood. He spoke on condition of anonymity as he was not authorized to brief journalists.

Video obtained by The Associated Press showed troops and militiamen marching alongside Humvees flying Iraqi military and Shiite militia flags in that northern neighborhood.

Tikrit, the capital of Salahuddin province, lies about 130 kilometers (80 miles) north of Baghdad. It is one of the largest cities held by Islamic State militants and lies on the road connecting Baghdad to Mosul. Retaking it will give Iraqi forces a major supply link to retake Mosul.

U.S. military officials have that said a coordinated military mission to retake Mosul, Iraq’s second-largest city, likely will begin in April or May and involve up to 25,000 Iraqi troops. But the Americans have cautioned that if the Iraqis are not ready, the offensive could be delayed.

Iraqi forces retook the town of Alam, which borders Tikrit, on Tuesday, sealing 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.