An infamous Dutch soldier turned ISIS fighter says the group has acquired chemical weapons once belonging to Syrian President Bashar al-Assad, resurrecting fears that what was supposed to be the regime’s destroyed program has instead fallen into jihadi hands.

“The regime uses chemical warfare on a regular basis these days, and nobody bats an eye—yet when [ISIS] captures it from them and uses it against them it’s all of a sudden a huge problem?” ISIS fighter Omar Yilmaz, 27, said in a Tumblr post. “Fight them the way they fight you.”

The post marks the first time a public ISIS figure has declared that the group obtained chemical weapons from the Assad regime. And it comes just days after the first series of suspected ISIS mustard gas attacks in northern Iraq and Syria.

On Tuesday, Kurdish forces said ISIS fired a homemade rocket filled with chemical weapons at peshmerga forces. In a suspected August 21 attack in the northern Syrian city of Marea, at least 25 people were contaminated. And on August 13, Kurdish officials in Iraq said 60 peshmerga were exposed to mustard gas in the northern Iraqi city of Makhmour.

Pentagon officials believe there is credible evidence that mustard gas could indeed have been used in the two August strikes.

Yilmaz’s August 31 post renews questions of ISIS’s source for several suspected chemical weapons attacks it orchestrated in northern Iraq and Syria. Did the Assad regime fail to fully destroy its chemical weapons arsenal? If Yilmaz’s claims are true, that would refute Pentagon claims that the group has developed its own rudimentary weapon.

Defense and intelligence officials told The Daily Beast on Wednesday that despite Yilmaz’s claims, they are still skeptical the weapons under ISIS control came from the Assad regime.

These officials noted that the recent attacks did not have the kind of impact they would expect to see from a state-sponsored chemicals weapons program. Attacks from such programs have the potential to kill thousands, as they did two years ago in the Damascus suburbs. These recent attacks instead injured scores.

Officials said they believe the weapons ISIS used are homegrown, noting the attacks have been rudimentary and that such weapons could be created by anyone with the right basic supplies. That is, the type of attacks believed to be carried out by ISIS did not require state-acquired weapons.

But critics note that the impact of the attacks could speak to how much state-acquired weapons have degraded. Others said ISIS could have state-created chemical weapons but not the munitions to disperse them effectively, weakening their impact. At the time Assad agreed to destroy his weapons, he did not control all the territory or facilities that held such weapons, still others asserted.

The most cynical of critics suggested that Assad could have purposely supplied ISIS with such weapons to perpetuate the narrative that he is confronting a far more ruthless foe than his regime.

Defense officials are dubious. Such weapons, if they still exist in Syria, “Assad is keeping for himself, in case he wants to use” them, one defense official retorted.

Either way, Yilmaz’s claims elevate the level of terror the group has sown in the region and the prospects that sophisticated chemical weapons are now part of its arsenal.

“I think [ISIS] is trying to convey several things. Its propaganda has been geared at intimidating enemies. This serves that purpose,” said Daveed Gartenstein-Ross, a senior fellow at the Washington, D.C.-based Foundation for Defense of Democracies. “And it wants to show its capability to would-be allies, to attract fighters.”

Yilmaz, as he is known, is a Dutch citizen of Turkish descent who first attracted attention roughly two years ago when photos emerged of the jihadist fighter wearing a Dutch military uniform. At the time, he was a facilitator for several other jihadist groups. His Instagram account depicting fighting in Syria, his prolific online presence, and his willingness to communicate with the West made him one of Europe’s highest-profile jihadists. Yilmaz reportedly first traveled to Syria after he was turned down for the Dutch’s military’s elite special forces.

In the last year he reportedly joined ISIS.

Last year, the mother of his onetime supposed 19-year-old bride, a Dutch woman raised Catholic before converting to Islam, retrieved her daughter from the Turkish-Syrian border. According to several postings online attributed to him shortly after she fled, he has since remarried.

In an October 2014 CBS News interview, Yilmaz said he felt that Syria was his homeland.

“We want Islamic law. We want our own rules,” he said in the interview from Syria, adding: “This fight never ends. This is our religion.”

The international push to rid Syria of chemical weapons began in the summer of 2013 after more than 300 people were killed in a chemical weapons attack in Ghouta, a rebel-controlled suburb of Damascus. The West believed Assad carried out the attacks, while the Syrian leader blamed opposition forces. President Obama had called the use of chemical weapons by the regime a red line, and the images of children convulsing after being exposed to chemical weapons created an international outcry. The U.S. appeared to be poised to launch strikes on Syria in response when the regime agreed to rid its nation of chemical weapons under a U.S.- and Russia-brokered agreement.

In August 2014, the Organization for the Prohibition of Chemical Weapons said it had verified that Syria had destroyed 1,040 tons of its Category 1 chemical weapons or munitions filled with chemicals that have no peaceful purpose.

But in May, OPCW reported its inspectors found traces of traces of sarin and VX nerve agent at a Syrian military research facility, suggesting the regime lied about destroying its arsenal or the extent of his stockpile.

— with additional reporting by Noah Shachtman