WASHINGTON -- The Islamic State militant group may possess chemical weapons that it has already used to extend its self-proclaimed caliphate, according to photos taken by Kurdish activists and examined by Israeli researchers.

The group, making gains in Iraq and Syria, may have captured chemical agents in Iraq in June and used them in July to kill three Kurdish fighters in the strategically important region of Kobani in northwest Syria, suggests a report released Sunday by the Global Research in International Affairs Center, a branch of the Interdisciplinary Center in Herzliya, Israel.

If verified, Islamic State's possession of unconventional weapons could make international efforts against it more urgent, and bolster claims that the world has not responded quickly or powerfully enough to the threat. The group, also known as ISIS, has intensified its effort to conquer Kobani over the past month, and battles there have attracted global attention as the region's defenders -- both Kurds and U.S.-backed rebels -- have urged international help.

Jonathan Spyer, author of the report, uses photographic evidence provided by Kurds in Kobani and a 2004 CIA report about the Iraqi chemical weapons production facility captured by ISIS in July to suggest that "on at least one occasion, Islamic State forces did employ some form of chemical agent, acquired from somewhere, against the [Syrian Kurdish forces] in Kobani." He said Israeli chemical weapons experts examined the Kurds' photographs. In response to questions from The Huffington Post, he declined to give their names.

"The probable possession by the Islamic State of a [chemical weapons] capability is for obvious reasons a matter of the gravest concern, and should be the urgent subject of further attention and investigation," Spyer says.

The report accuses the Islamic State of using chemical weapons in a July 12 battle in an eastern part of Kobani during a previous offensive into the Kurdish enclave. The site of the battle is now controlled by ISIS. Spyer cites signs of a chemical weapons attack mentioned by the health minister of Kobani to the Lebanese online news outlet Al-Modon four days after the attack. In Spyer's telling, the minister said that the corpses of three Kurdish fighters exhibited "burns and white spots … [that] indicated the use of chemicals, which led to deaths without any visible wounds or external bleeding." The bodies had not been hit by bullets, the minister added.

Spyer's report includes gruesome photographs of the bodies now circulating on social media alongside appeals for more help for the Syrian Kurds in Kobani.

In emails to The Huffington Post, Spyer said he had been given the pictures by Kurds in Kobani, whose identities he could not reveal. He said he takes them seriously because they were provided to him weeks ago -- not to boost the case for international help to Kobani, but to spur an investigation by international authorities.

A reported chemical weapons attack by the government of Syrian President Bashar al-Asaad received broad international attention last year. Despite an international deal to destroy Asaad's weapons, the regime revealed recently that it still has previously undisclosed chemical weapons facilities.

"Because the area in question is now controlled by [the Islamic State], and the Kurdish enclave in Kobani may itself shortly cease to exist, we decided that this goal [of an international investigation] was no longer feasible and so decided to publish the pictures," Spyer wrote to The Huffington Post.

A key question is whether the alarm could have been raised earlier. Iraq informed the international community that ISIS had captured a massive former chemical weapons facility at Muthanna, northwest of Baghdad, almost a month after the event. The country's ambassador to the United Nations, Mohamed Ali Alhakim, flagged two bunkers at the complex -- one containing sarin-filled rockets, the second with mustard-contamined artillery shells.

Speaking about the Islamic State's attack on the facility, State Department spokeswoman Jen Psaki said over the summer that the two bunkers "don't include intact chemical weapons ... and would be very difficult, if not impossible, to safely use this for military purposes or, frankly, to move it."

Spyer writes that a CIA investigation of the facility in 2004 suggested the presence of materials that ISIS could move to its capital of Raqqa.

A State Department official told The Huffington Post on Wednesday that the agency was "aware of the reports but cannot confirm details."

"We take allegations of chemical weapons use very seriously and are seeking more information" on the potential ISIS attack, said the official, who asked not to be quoted by name.

Jean Pascal Zanders, a chemical weapons expert who runs the blog The Trench, argued in a recent post that it's unlikely that ISIS could use the chemical weapons residue it captured, because the sarin and mustard gas degrade over time.

Asked about that reasoning, Spyer said the Israeli experts who had seen the photographs believed the Kurdish victims had been exposed to mustard, which takes longer to degrade than sarin. Spyer added that it is impossible to reach a firm conclusion without further investigation.

RT, formerly known as Russia Today, showed the same pictures to a British chemical weapons expert, Hamish de Bretton-Gordon. Bretton-Gordon told the channel that if genuine, the photographs appear "consistent with a blister agent, like mustard."

Max Abrahms, a professor at Northeastern University who studies counterterrorism, told The Huffington Post chemical weapons may help ISIS enforce compliance among communities like the Syrian Kurds, and scare rival militant groups. He said the finding was unlikely to significantly change the White House calculus.

"If they wanted to use this as sort of a rallying cry for putting in U.S. troops on the ground, they might be able to," Abrahms said. "But I don’t think they want to."

This article has been updated to include the State Department's comment.