Islamic State militants attacked Syrian army troops with mustard gas in an offensive against a Syrian military airport in the eastern province of Deir al-Zor that borders Iraq, state media said late on Monday.

Syrian state media did not disclose how many casualties were sustained in the latest drive by the hardline fundamentalist Sunni militants to capture the heavily defended airport located south of Deir al-Zor city, whose main neighbourhoods are under the militants’ control.

“The terrorists fired rockets carrying mustard gas,” a statement said on state-owned Ikhbariyah television station.

Deir al-Zor is a strategic location. The province links Islamic State’s de facto capital in Raqqa with its fighters in Iraq.

Reuters could not independently verify the media reports.

Amaq news agency, which is close to the militants, had earlier said Islamic State fighters had launched a wide-scale attack on Jufrah village near the airport in which it said two of its suicide bombers rammed their vehicles into army defences causing “tens of dead”.

“The battles continue on more than front and posts and we pray to Allah (God) victory for his Mujahdeen (holy warriors),” an official statement by the militants said.

The UK-based Syrian Observatory for Human Rights, a monitor which tracks violence across the country, said the militants had advanced with heavy aerial strikes aimed at repelling their offensive.

The Syrian army backed by heavy Russian air strikes was able last January to drive back the hardline militants from several villages near the airport but has so far failed to dislodge them.

Separately, the observatory said fighting flared on several frontlines in the major northern city of Aleppo which is divided between government and rebel-held sectors.

Rebel shelling of Kurdish YPG outposts in Sheikh Maqsoud neighbourhood caused several casualties, the monitor said.

The Syrian army had earlier said that at least 400 al-Qaeda-affiliated Nusra Front-led militants fully equipped with heavy arms staged a major attack on army outposts in the Aleppo countryside.

The army statement also said at least eight civilians were killed in mortar attacks by rebels on residential areas of Sheikh Maqsoud with scores injured.

A fragile “cessation of hostilities” truce has held in Syria for over a month as the various parties to the conflict try to negotiate an end to Syria’s civil war.

But the truce excludes Islamic State and the al Qaeda-affiliated Nusra Front. Air and land attacks by Syrian and allied forces continue in parts of Syria where the government says the groups are present.

Reuters