A fire set by Isis (Islamic State) militants at a sulphur mine near the city of Mosul in recent days sent plumes of noxious gases over the battlefield, making hundreds of civilians sick and forcing Iraqi and US troops to wear protective masks, health and military officials said.

Firefighters were still struggling to put out the blaze at the Mishraq sulphur mine, about 40km southeast of Mosul, according to Colonel Abdulrahman Al-Khazali, a spokesman for Iraqi Federal Police who visited the mine.

Officials gave no indication that the fumes had interrupted a broader five-day-old offensive to capture Mosul from the militants. But the smoldering sulphur added to the list of unconventional weapons - including oil fires, armour-plated car bombs and exploding drones - the militants have deployed in an effort to slow the march of Iraqi forces towards the city.

In another attempt to distract their opponents, dozens of Isis fighters staged a bold raid on government buildings and police positions in the northern city of Kirkuk, east of Mosul, on Saturday. The attack was largely contained by yesterday, but at least 80 people, mainly Kurdish security forces, had been killed during the incursion, a Kirkuk police official said.


Soldiers were seen wearing surgical masks about 25km south of the fires, where a highway and surrounding villages were blanketed in a dull grey haze. Several oil fires burned nearby, obscuring the view of the horizon with a curtain of black smoke.

In recent days, shifting winds began blowing the noxious fumes over US troops stationed at a forward staging base near Mosul, according to American officials.

Although much of the mine was burned during the 2003 US-led invasion of Iraq, about 10 per cent of the sulphur still remains.

The fires added to concerns that the militants might use crude chemical weapons as Iraqi troops close in on Mosul. Isis is believed to have what a senior American official described as a "very rudimentary chemical weapons programme" in the city that includes mustard and chlorine gases.

"It can easily go from rudimentary to more sophisticated," said the official. "We have worked hard to identify it," he said, speaking of the programme.

Tens of thousands of Iraqi and Kurdish peshmerga soldiers are involved in the Mosul offensive, which started last week following months of operations in the countryside around the city. They have steadily advanced across swaths of desert and into small hamlets as they draw within kilometres of the city.

Despite the progress, the forces are at least days, if not weeks, from the city itself. The fighting is expected to be heaviest there, where Isis has focused the majority of its defences, US officials said.

Keeping up the momentum of the operation is seen as critical to maintaining the alliance of disparate forces with often-competing interests that are cooperating, for the moment, to recapture Mosul.

Turkey, which maintains a contingent of roughly 500 troops near Mosul, has insisted on a role in the offensive. Iraqi officials say the Turkish troops, who have been training Sunni forces at a base there since December, are in the country without permission.

Iraqi Prime Minister Haider al-Abadi again rejected the possibility of Turkish involvement in the current campaign - just a day after US Defence Secretary Ashton Carter announced that he had reached an agreement "in principle" with Turkish officials that would allow Turkey to participate in the battle.

Tensions over Mosul have escalated in recent weeks into an incendiary war of words between Abadi and Turkish President Reccep Tayyip Erdogan. The Obama Administration has struggled to mediate between the two countries, both close allies of the US and seen as playing critical roles in the fight against Isis.

Meanwhile, Jason Finan, a 34-year-old chief petty officer, is the first American killed in the current battle for Mosul.

Finan is survived by his wife, Chariss, and their 7-year-old son, of Imperial Beach, California, the San Diego Union-Tribune reported.

The Pentagon said Finan died on Friday, after sustaining wounds in an improvised explosive device blast.

Finan, of Anaheim, California, was assigned to the Navy's Explosive Ordnance Disposal Mobile Unit 3, based in Coronado, California. Finan enlisted in the Navy in August of 2003 and had earned several commendations.

Finan was part of a US team advising and assisting peshmerga. His is the fourth US combat death in Iraq since military operations against Isis began in August 2014.