The Pentagon said Friday that it also was looking into reports of another possible mustard gas attack this week on Kurdish fighters.

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WASHINGTON — The Islamic State group is suspected of using chemical agents — said by some U.S. officials to be mustard gas — in an attack on Kurdish fighters in northern Syria two weeks ago, U.S. officials said Friday.

If authenticated, the chemical attack would be an escalation of the more than yearlong conflict under way in Iraq and Syria, and could increase pressure on the Obama administration to intervene more forcefully in the war against the Sunni militant group.

The Pentagon said Friday that it also was looking into reports of another possible mustard gas attack this week on Kurdish fighters — this one in Makhmur, Iraq.

Kurdish media reports on Thursday quoted local officials saying that mortar rounds fired at Kurdish positions in Makhmur may have contained mustard gas because the wounds to injured peshmerga fighters were different from those in a conventional attack.

“We’ve seen those reports, and we’re taking them seriously,” Col. Patrick S. Ryder, a spokesman for the U.S. Central Command, said in a telephone briefing with reporters Friday.

He added, “At this point, we really don’t know what, if anything, may have been used.”

Ryder declined to say whether the United States has sent independent inspectors to verify the attacks.

The question is complicated by history. Kurds have suffered chemical attacks in the past, but since 2002 various Kurdish officials seeking Western support have highlighted stories of chemical attacks and chemical weapons stocks that were never confirmed.

But on Friday two U.S. officials said the defense and intelligence community had come to believe that mustard gas was used two weeks ago in the northern Syria attack, which injured several Kurdish fighters.

That view, one official said, had prompted assessments that the Makhmur report this week was “plausible.”

The Kurdish claims were not independently verifiable, and further details of the purported attacks were scarce.

At least hundreds of old and often corroded mustard artillery shells remained in Iraq after the United States invaded in 2003. By 2004, Sunni insurgents had begun occasionally using the shells in improvised bombs against U.S. forces.

But Kurdish claims of mortar shells containing mustard gas being fired, as opposed to being used in hidden explosives, were new and do not match past insurgent patterns.

On Thursday, the Kurdish media outlet Rudaw quoted a Kurdish commander, Muhammad Khoshawi, as saying that on Wednesday night “at least 45 mortar rounds were fired at our positions, which we believe were loaded with chemicals, since the wounds are different.” H

e said that evidence had been sent for examination but that there were no conclusive results.

The attack this week in which the Islamic State may have used mustard gas was first reported by The Wall Street Journal.

For President Obama, a confirmed use of chemical weapons by the Islamic State group could increase the pressure for him to move more forcefully against the militant group.

Obama has said that the use of chemical weapons would cross a red line that could force U.S. action.