Also yesterday, Australia confirmed it will send an extra 300 troops to Iraq to help train the local military for the fight against Isis.
"This decision marks the next phase of Australia's contribution to the international coalition to disrupt, degrade and ultimately defeat the Daesh ... death cult," Prime Minister Tony Abbott said.
About 170 Australian special forces personnel have been training Iraqi and Kurdish military since September and an air force task group has been involved in bombing raids.
Yesterday the Iraqi air force released video of what were claimed to be government jets striking Isis positions in advance of the attack.
But in a key gesture, the US-led coalition that is supporting the Baghdad Government from the air said it was not involved. That might indicate unease at the strategy, which includes heavy use of Shia forces to retake a largely Sunni area. Many supporters of the Government have been calling for revenge on the local population if and when Tikrit is taken.
It was the site of the worst individual sectarian massacre of recent times in the Middle East, when Isis announced it had killed 1700 Shia soldiers it captured at Camp Speicher, the area's main military base, as it swept through central and northern Iraq last June.
Pictures of the massacre, in which Sunni captive soldiers were spared but long lines of Shia men were led out and shot in front of open graves, horrified the world.
This week's government offensive had been signalled for days in advance, and was announced by Haider al-Abadi, Iraq's Prime Minister, on state television.
He urged Sunni Muslims and tribal leaders in Tikrit's Salahuddin Province who had sided with Isis to switch sides before it was too late.
"This is their last chance," he said. "If they insist on staying on their wrong path they will receive the fair punishment they deserve because they stood with terrorism."
Tikrit, about 160km north of Baghdad, has long been a symbolic battleground for all participants in Iraq's conflict.
Many of its citizens remained loyal to Saddam Hussein even after he fell. His grave is nearby, and many of his supporters, including former army officers, have joined Isis either through genuine belief in the jihadists' goals or as an alliance of convenience among Sunni fighters against the Shia-dominated Government, which is backed by Iran as well as the West.
Its population long complained of discrimination at the hands of the police and army after Saddam's fall, and many people welcomed Isis's arrival in June.
In what may now confirm many of their worst fears, the "hard man" of the Iranian military, General Qassem Suleimani, leader of the Quds Force, which oversees the Revolutionary Guard's international operations, seems to be taking a personal role in the fighting. Iranian and Iraqi news outlets said he was in Tikrit directing operations.
Iraq operation
• Additional 300 Australian regular troops to be deployed by May.
• Joint mission with NZ: train Iraqi soldiers in basic weapon skills, operations, medical and logistics.
• NZ sending 143 soldiers.
• US, Spain, Italy, Germany, Denmark and Netherlands also sending trainers.
• Two-year mission, reviewed after 12 months.
• Troops to be based at Taji, about 30km north of Baghdad.
• 170 Australian special forces already in Iraq due out in September.
