Hundreds of people in Iraq are being treated for the effects of toxic gases after a sulphur plant was set alight in fighting with the Islamic State militants.

The US military said IS fighters set the plant on fire earlier this week, as they fled an advance by pro-government forces on their Mosul stronghold, reported BBC.

On Saturday, US soldiers at a base near Mosul donned protective masks as wind blew smoke towards them. Reuters said another 1,000 people were being treated for breathing problems.

An Iraqi commander, Qusay Hamid Kadhem, told AFP news agency two civilians had died from the fumes and "many others" had been injured.

Meanwhile, advancing Iraqi forces entered the town of Qaraqosh, 32km south of Mosul, the IS capital. Qaraqosh, Iraq's largest Christian town before the war, is said to be largely empty but IS has laid landmines on the approaches to Mosul.

The militants have been attacking with suicide bombers elsewhere, driving vehicles laden with explosives at high speed towards government lines.

Qayyarah, where the plant was set alight, acts as the main US hub for supporting the Iraqi government offensive to drive IS out of their Mosul stronghold, said the report.

The fire began two days ago, when IS fighters reportedly set the sulphur plant alight in Mishraq, south of Mosul, it added.

A similar fire at the Mishraq plant in 2003 burnt for weeks, sending huge amounts of sulphur dioxide into the air. It caused respiratory problems for local people and damaged the environment.

Meanwhile, US Defence Secretary Ash Carter made an unscheduled visit to Baghdad on Saturday after talks with Turkish leaders in Ankara on Friday on how they might play a part in the Mosul operation.