The US army detected sulfur mustard on the fragments provided to them by the Kurdish fighters, Brigadier General Kevin Killea told reports in Pentagon on Friday.

"It is very important to understand here that that is a presumptive field test and it is not conclusive, and what those results tell us is merely the presence of that chemical," said Killea, chief of staff for American military operations in Iraq and Syria.

The testing is not definite proof of chemical weapons use, and the more extensive probes are underway to get the full make-up of the chemicals, according to the general.

At the same time, Killea stressed that sulfur mustard is a Class 1 chemical agent, which means it has few uses outside chemical warfare.

Suspicions of gas attacks

US officials have been looking into reports accusing "Islamic State" (IS) of using chemical weapons. According to some sources, the jihadists used chemical weapons in an attack on Kurdish forces in the Iraqi town of Makhmur last week. Similar accounts were also publicized in July.

The grenade fragments tested by the US army come from a mortar round fired during the attack on Makhmur, which were handed over to US forces in the region.

In his Friday statement, Killea warned that there might be questions on the chain of custody of the evidence, and said that it would take weeks to get more information.

It also was not immediately clear how IS fighters could have obtained chemical weapons. In 2013, Russia and the United States launched a joint effort that led to the removal or destruction of chemical weapons stockpiles in Syria.

dj/kms (AP, Reuters)