Australia announced Tuesday it will send another 300 troops to Iraq in a joint mission with New Zealand to help train local forces fighting to reclaim territory seized by the Islamic State group.
The decision follows Wellington last week deciding to deploy some 140 soldiers in a non-combat role to boost the Iraqi military's ability to battle the jihadists.
Prime Minister Tony Abbott said the decision followed a formal request from the Iraqi and US governments with the mission intended to span two years.
"I want to stress that we haven't taken this decision lightly. Ultimately, it is Iraq that must defeat the death cult (Islamic State) but we do not want to leave the Iraqis on their own," he told reporters.
"We are naturally reluctant as a peace-loving people to reach out to far-away conflicts but, as we know, this conflict has been reaching out to us for months now."
The move came as some 30,000 Iraqi troops and militia backed by aircraft pounded jihadists in and around Tikrit in the biggest offensive yet to retake one of the Islamic State group's main strongholds in the country.
The operation is the broadest since IS overran swathes of the country last year and is seen as a step towards the liberation of Mosul, the jihadists' main hub in Iraq.
Some 170 Australian special forces are already in Iraq helping to train government troops, and Abbott said it was in his country's national interest to bolster their presence.
He said about 100 Australians were fighting with Islamic State and other terrorist groups in Syria and Iraq.
"About another 150 here at home are supporting these extremists so this commitment is a matter of domestic as well as international security, and I stress this is absolutely and utterly in Australia's national interests to do this," he said.
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- 'Inside the wire' -
Earlier this month, Abbott warned of a long era of heightened threats from "home-grown" Islamic State-inspired extremists and announced new security measures, including revoking citizenship for dual-nationals linked to terrorism.
It followed a series of counter-terrorism raids in Sydney and Brisbane since September and a string of incidents, including a December siege in a Sydney cafe by a self-styled cleric that left the gunman and two hostages dead.
The Australian and New Zealand troops are to be based at a military base in Taji, north of Baghdad, from May.
"I want to stress that this is a training mission, it's not a combat mission. It's inside the wire, it's not outside the wire," added Abbott.
"But it is absolutely vital for our national security because I said at the beginning this Daesh (Islamic State) death cult is reaching out to this country as well as to the people of Iraq and Syria.
"The protection of Australia requires work abroad as well as at home and this government will never shirk the tasks that are necessary to keep the Australian people safe."
Asked whether it would be Australia's last contribution to the Iraq mission, he said: "It would be wrong of me to say that this is the last that we will do here."
Since August 2014, the US military -- along with allies including Australia -- has been conducting a campaign of air strikes against the jihadists in Iraq and Syria.
Australia's role is restricted to aerial support, training, advice and intelligence.
Australia contributed about 2,000 troops to the US-led coalition's war in Iraq from March 2003, until they withdrew in 2009. None died in combat or on operational duty during that deployment.