U.S. intelligence agencies reportedly believe the Islamic State used chemical weapons in Syria on Kurdish forces at least two weeks before the group may have used it in Iraq, U.S. officials said on Friday.

The reports are fueling concerns that ISIS has acquired an arsenal of banned chemicals that could escalate fighting in the region.

“It could be a pattern,” a senior U.S. official with knowledge of the intelligence told The Wall Street Journal.

U.S. agencies have believed that the Islamic State aspired to obtain chemical weapons, such as mustard gas, but officials said it was premature to immediately connect the three attacks because the agents involved in the Iraq incidents were still in the process of being analyzed.

Officials told the journal the agencies confirmed through tests that mustard gas was used in northern Syria in July, making it the first known attack by ISIS using banned weapons.

Pentagon officials believe the terror group used chemical weapons against Kurdish Peshmerga fighters in northern Iraq Wednesday, Fox News learned. One official who had seen the latest intelligence reports from the region told Fox News Thursday that the victims had “blisters” that matched the symptoms of other victims of mustard gas.

U.S. intelligence agencies are still trying to determine the origin of the chemicals used in the attacks. A U.S. official tells Fox News the mustard agent was "homemade" and not from Syria.

Senior officials told The Wall Street Journal on Thursday that ISIS may have obtained the mustard gas in Syria, whose Damascus government admitted to having large stockpiles of the chemical when it agreed to give up its chemical weapons arsenal in 2013. However, some of the intelligence community doesn't think ISIS has a large chemical weapons cache.

According to the Journal, U.S. intelligence officials believed ISIS had seized a small amount of mustard gas prior to Wednesday's reported attack, though that assessment had not been made public. Now, the paper reports, officials fear that ISIS could discover more hidden caches of chemical weapons elsewhere in Syria as troops loyal to Bashar Assad lose ground in the country's bloody civil war.

Kurdish leaders said on Friday that ISIS had fired 45 mortars carrying chemical warheads, according to the website of a Peshmerga spokesman, Secretary-General Jabar Yawar.

On Thursday, Alistair Baskey, a spokesman for the White House's National Security Council, said the U.S. 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.

Iowa Sen. Joni Ernst said the alleged use of chemical weapons should prompt the White House to review whether its making the right approach in Syria.

“I think our administration needs to go back to the drawing table and take another look at this—not only are they using it on the Kurdish Peshmerga, but what’s to say they don’t start using it against American soldiers as well?” Ernst told the newspaper.

Following a chemical weapon attack on a suburb of the Syrian capital of Damascus in 2014 that killed hundreds of civilians, the U.S. and Russia mounted a diplomatic effort that resulted in 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.

Word of the White House's probe into possible chemical weapons use by ISIS came as President Barack Obama was vacationing with his family in Martha's Vineyard in Massachusetts. Also on Thursday, IS militants 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.

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Fox News’ Lucas Tomlinson and the Associated Press contributed to this report.