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

If authenticated, the chemical attack would be an escalation of the more than yearlong conflict underway 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 news 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 pesh merga 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 United States 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.”