Iraqi troops say battle to retake city will not be easy, with areas “littered with bombs and booby-traps”.
The US defence department has denied claims that it killed two Iranian advisers in drone strikes in Iraq earlier this month, telling Al Jazeera it had no role in the area during the time of their deaths.
Pentagon officials dismissed allegations made by Iran on Monday that US-led strikes killed Ali Yazdani and Hadi Jafari, two members of Iran’s Revolutionary Guard Corps (IRGC), on March 23 near the city of Tikrit.
“Coalition forces initiated airstrikes near Tikrit on March 25, two days after the alleged incident occurred and no airstrikes were conducted in or near Tikrit on March 23,” the department told Al Jazeera.
“Additionally, we have no information to corroborate claims that coalition airstrikes killed two IGRC members.”
A major offensive, backed by US airstrikes, is underway to retake Tikrit from the Islamic State of Iraq and the Levant (ISIL) group.
However, in recent days Iraqi forces have struggled to advance on the city after Shia militias boycotted the latest offensive after the US, who did not initially take part in the offensive, joined the operation.
Shia militias, who receive backing from Tehran, have repeatedly said they do not need US support to drive ISIL from Tikrit despite the Baghdad government requesting the raids.
A US-led coalition began airstrikes against the ISIL in Iraq in August, while Iran has been advising and supporting the Shia militias that have played a key role in ground operations alongside Iraq’s military.
However, Washington and Tehran, which remain deeply divided on a host of regional issues, insist they are not coordinating with one another on the battlefield.