The Islamic State group has launched a renewed push to seize Iraq's main oil refinery north of Baghdad, battling security forces backed by air support, authorities and witnesses said.

At least three Iraqi security personnel were killed, and seven others were injured in the fighting on Sunday, in which fighters attacked the Baiji refinery from three sides, sources told Al Jazeera.

The attack on Sunday came as Iraq's next-door neighbour Iran reiterated its support for the new Shia-led government of Prime Minister Haider al-Abadi, with a visit from Iranian Foreign Minister Javad Zarif.

The fight over the oil field started overnight and continued into the next day, according to news reports.

The Islamic State fighters have repeatedly sought to overrun the refinery, which once accounted for about 50 percent of Iraq's supplies of refined oil products.

While they have previously managed to enter the refinery compound, security forces were able to push them back at this time.

The unrest has hit northern oil production and shipments, but Iraq's massive southern fields and export terminals remain unaffected.

Iran-Iraq alliance

The fighting on Sunday comes as Iranian Foreign Minister Javad Zarif began a two-day visit in Baghdad, which is trying to fend off the Sunni-led rebels.

Zarif met his Iraqi counterpart Hoshyar Zebari as well as the prime minister-designate with the issue of the Iran-Iraq border demarcation also on the agenda.

Iran, which like Iraq is predominantly Shia, was a longtime backer of the outgoing Prime Minister Nouri al-Maliki, before dropping its support of the divisive leader in favour of Abadi.

Tehran has called for Iraqi parties to unite in the face of the Islamic State, which has seized large swathes of the country and says it is providing advice to Iraqi Kurds fighting the armed group.

Dozens of people have been killed in Iraq in the past 24 hours with violence near the Iranian border getting worse.

Zarif said Iran will support Abadi, and try to help the country find stability.

During a press conference with Zebari, Zarif also denied reports that Iranian troops have been deployed across the border to fight the Islamic State in Iraq.