BAGHDAD: Islamic State, the Al-Qaeda spin-off that seized wide swathes of Iraq almost unopposed last month, has released a video warning Iraqi soldiers who may still have some fight in them that they risk being rounded up en masse and executed.

Iraq’s Army unraveled when the insurgents staged a lightning advance through northern towns and cities, building on territory their comrades captured earlier in the west of the country, a major OPEC oil producer.

Thousands of soldiers fled, prompting Iraq’s top cleric to call on compatriots to take up arms against the radical faction.

The 30-minute video clip sheds light on what tactics the Islamic State is likely to employ as it presses ahead with its campaign.

After sweeping through a town with quick-hit raids, the militants are filmed standing over dozens of terrified, handcuffed Iraqi soldiers.

One fighter mocks a soldier for wearing civilian clothes over his uniform out of fear of being identified and killed. He pleads for mercy, to no avail. Then dozens of soldiers are led to a sandy desert pit. They are executed one by one — bullets from AK-47 assault rifles pumped into their heads.

Not satisfied that all are dead, an Islamic State fighter makes one last round, opening fire again, one by one.

Others are led to the edge of a river. Each one is shot in the head with a pistol and then shoved in, the executioner standing in a large pool of blood. The mission begins with an Islamic State commander firing up militants with promises that paradise in heaven awaits them if they take the city of Samarra.

The footage features night-vision sequences, then shows fighters moving into a city in flat-bed trucks, and US-made Humvees seized during their surge through the north last month.

The Islamic State convoys filmed include small tanks and heavy machine guns transported on trailers.

Some fighters are on foot, darting toward government buildings on sandy roads. As the Islamic State gains ground small units in trucks pull up beside vehicles during daytime and open fire on passengers who lose control of their vehicles.

Several militants walk up to a van and empty their AK-47s through the windows, to make sure the job is done.

Then the camera focuses on Iraqi security forces in watchtowers. One by one they are picked off by Islamic State snipers, who seem to avoid heavy clashes and rely instead on quick, small operations combined with psychological warfare.

An insurgent can be heard weeping in joy as he declares that Samarra now belongs to the group.

On Tuesday, Islamic State militants blew up a strategic bridge connecting Samarra to the town of Tikrit to the north, severing the main route and a tunnel beneath it that was used by Iraqi military forces, a senior local police official said.