The Islamic State Is Believed to Have Used Chemical Weapons Against the Kurds

Kurdish peshmerga forces in northern Iraq were attacked with rockets filled with what they say was a chemical agent on Tuesday — believed to have been chlorine and launched by Islamic State (IS) militants.

"We have indications that there was an attack with chemical weapons," a German defense ministry spokesman told AFP, adding that the incident left many Kurdish fighters suffering from "respiratory irritation."

US officials described the reports to the BBC as "plausible," and told the Wall Street Journal that the attack likely used mustard gas.

Earlier on Thursday, Kurdish officials said their forces were attacked near the town of Makhmour, not far from Erbil. Germany's military has been training the Kurds in the area, and the German officials said some 60 Kurdish fighters had suffered breathing difficulties from the attack — a telltale sign of chemical weapons use.

Confirmation of chemical weapons use by IS would mark a dramatic turn in the international efforts to rout the extremist group from the roughly one-third of Iraq and Syria that it controls.

"It's hard to be sure about the latest reports," Olivier Lepick, a research fellow at the Foundation for Strategic Research and an expert in chemical weapons, told VICE News. "There have been rumors in the past about IS using chemical weapons," Lepick added, noting that previous reports had sometimes turned out to be false. "We have to be cautious and wait for the results of an investigation," he said.

"What is interesting, however, is the way in which the attack was carried out," said Lepick. Unlike previous rumors of chemical attacks by IS, "Tuesday's attack was not carried out with a bomb, but with Katyusha multiple rocket launchers from Russia."

Related: Islamic State Claims Responsibility for Baghdad Bombing That Killed 62

This could mean two things, according to Lepick. On the one hand, IS may have acquired the technology to replace the classic explosives contained in the rockets with chemicals — a feat Lepick said would only involve "limited" resources. On the other hand, IS may have acquired chemical weapons left over after the fall of Saddam Hussein in Iraq.

"IS is obviously interested in carrying out chemical attacks," said Lepick, adding that group had already tried to carry out chlorine attacks. "It's very easy to get your hands on a ton of chlorine and these types of attacks only require limited technical resources."

The US is also investigating whether the Islamic State used chemical weapons, the White House said on Thursday.

Alistair Baskey, a spokesman for the White House's National Security Council, said the US is taking the allegations "very seriously" and seeking more information about what happened. He noted that IS had been accused of using such weapons before.

"We continue to monitor these reports closely, and would further stress that any use of chemicals or biological material as a weapon is completely inconsistent with international standards and norms regarding such capabilities," Baskey said in a statement.

Although the US and its coalition partners are mounting airstrikes against IS, they are relying on local forces like the Kurds, the Iraqi military, and others to do the fighting on the ground against the might of the well-funded and heavily armed extremist group.

Watch the VICE News documentary, Kurds Assert Control of Hasakah: The Battle for Rojava (Dispatch 3) here:

At the United Nations, US Ambassador Samantha Power said the country was speaking with the Kurds who had made the allegations to gather more information. "I think we will have to again move forward on these allegations, get whatever evidence we can," Power said.

She added that as a result of earlier chemical weapons use by the Syrian government, the US and its partners now have advanced forensic systems to analyze chemical weapons attacks. She said anyone responsible should be held accountable.

Related: Outgoing US Army Chief Urges Considering Boots on the Ground In Islamic State Fight

Similar reports of chemical weapons use by IS had surfaced in July. But it's unclear exactly where the extremist group may have obtained any chemical weapons.

Following a chemical weapon attack on a suburb of the Syrian capital of Damascus in 2014 that killed hundreds of civilians, the US and Russia mounted a diplomatic effort that resulted in Syrian President Bashar Assad's government agreeing to the destruction or removal of its chemical weapons stockpiles.

But there have been numerous reports of chemical weapons use in Syria since then — especially chlorine-filled barrel bombs. The Organization for the Prohibition of Chemical Weapons, the global chemical weapons watchdog, has been investigating possible undeclared chemical weapons stockpiles in Syria.

On Thursday, IS also claimed responsibility for a truck bombing at a Baghdad market that killed 67 people in one of the deadliest single attacks there since the Iraq War.

Related: Kurds Assert Control of Hasakah: The Battle for Rojava (Dispatch 3)

The Associated Press contributed to this report.