The United Nations' refugee agency has launched a major aid operation to get supplies to more than half a million people forced from their homes by militants in northern Iraq.

A four-day airlift of tents and other items will begin tomorrow.

Road convoys from Turkey and Jordan will start over the next 10 days.

The news of the operation comes as sources suggest that fighting has resumed at Mosul Dam after it was announced earlier that Iraqi and Kurdish forces had reclaimed control of the site.

Iraqi and Kurdish forces have been taking on Islamic militants for control of the strategic site which supplies water and power to millions of people down the Tigris river valley.

US fighter jets and drones have been attacking Islamic State (IS) targets as they try to help push back the Sunni extremists who have taken over large parts of the north and west of the country.

IS seized the dam several weeks ago but President Barack Obama announced on Monday that Kurdish fighters had regained control of the hydro-electric dam with the help of American airstrikes. Obama called recapturing the dam a "major step forward".

It had been feared that IS would cut pipes and cables or blow up the structure, causing huge loss of life and extensive damage along the Tigris valley.

Army spokesman Lt Gen Qassim al Moussawi said at least 170 bombs had been dismantled around the site but warned many more remain.

Mosul Dam. Photo: U.S. Army Corps of Engineers - Northwest Division

There is also fierce fighting near the centre of Tikrit, the hometown of former Iraqi leader Saddam Hussein.

Tikrit was seized by IS earlier in the year but Iraqi forces as well as Shia militias are trying to drive the insurgents out.

Islamic State warn US against airstrikes

Meanwhile, the IS militant group has warned the United States in an internet video it will attack Americans "in any place" if the airstrikes against its fighters continue.

The message includes a photograph of an American who was beheaded during the US occupation of Iraq, and also American troops being shot by snipers.

It carries a warning statement, on screen in English, "we will drown all of you in blood".

The US president said IS fighters remained "a threat to Iraq and the entire region" and said Iraqis "must reject them and unite by pushing them out of lands they have occupied".

"They claim to represent Sunni grievances but they slaughter Sunni men, women and children," he said, adding limited military missions would continue.

Unlike al Qaeda, IS has, to date, focused on seizing land in Iraq and Syria for its self-proclaimed caliphate, rather than attacking Western targets.

Originally published 7:30am