WASHINGTON, D.C. — The highest-ranking military officer in the U.S. confirmed that the Islamic State (ISIS/ISIL) used a potentially deadly “sulfur-mustard blister agent” to target a base in northern Iraq that houses American troops. This marks the first recorded chemical attack on the United States since the jihadist group conquered swaths of Iraq in 2014.

“We assess it to be a sulfur-mustard blister agent,” Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, told the Senate Armed Services Committee Thursday.

“It wasn’t particularly effective but it was a concerning development,” he added. No one was killed or injured in the attack, which occurred Tuesday evening.

The U.S. military is adequately prepared to protect themselves against chemical weapons, declared the general, noting that the troops have detection and decontamination equipment in their arsenal.

Gen. Dunford pointed out that the U.S.-led coalition has carried out nearly 30 airstrikes targeting ISIS’ “emerging” chemical weapons infrastructure.

He did not elaborate further on Tuesday’s attack.

“Earlier this month the Pentagon said it struck a pharmaceutical factory in Mosul that had been converted by the Islamic State into a chemical weapons factory,” reports USA Today. “The U.S. government has previously accused the Islamic State of using chemical weapons.”

Tuesday’s attack comes nearly three months after a top Kurdish commander in northern Iraq said that the Islamic State was stepping up its chemical weapons attacks in his region.

Gen. Bahram Arif Yassin, a top commander of the Peshmerga fighters from northern Iraq’s Kurdistan Regional Government (KRG), sounded the alarm on the chemical attacks in an exclusive interview with Breitbart News.

He made those comments a few months after a top Islamic State jihadist captured by the U.S. military in northern Iraq warned that the terrorist group was planning to use chemical weapons, namely mustard gas, against the United States military and its allies.

The captured terrorist was identified as the chief of the Islamic State’s recently established branch for the research and development of chemical weapons. He had worked for Saddam Hussein’s now-dissolved Military Industrialization Authority, where he specialized in chemical and biological weapons.

On Tuesday, the chemical-laden shell landed at a military facility in northern Iraq where U.S. military advisers are helping their Iraqi counterparts prepare for an upcoming offensive to retake Mosul, the last major stronghold held by the Islamic State in Iraq, notes USA Today.

The base is reportedly located in Qayyarah, 25 miles south of Mosul, Iraq’s second-largest city found in Sunni-majority Nineveh province, where Arabs, Kurds, and other minority groups have competing claims to territory.

The Islamic State has already carried out several chemical attacks in Syria, which borders Nineveh province. The jihadist group has also launched numerous chemical attacks in Iraq, primarily targeting the northern part of the country and mainly using chlorine and sulfur mustard, according to experts.

Although the two agents are considered poor weapons by modern standards, the Islamic State launched two chemical attacks in northern Iraq in March, killing nearly 600 people and wounding at least 100.

“As Daesh [ISIS] loses ground, they are becoming more desperate and, as a result, we have seen an increase in chemical attacks across Kurdistan,” Iraq’s Kurdish Gen. Yassin told Breitbart News in June.

At the time, the Kurdish commander’s brigade was operating at an anti-Islamic State frontline near Bashiqa, a Kurdish-controlled town about 20 miles north of ISIS-held Mosul.

In March, U.S. Special Forces captured an Islamic State jihadist in Nineveh province identified as the terrorist group’s top chemical weapons emir.

As of earlier this year, ISIS had mounted at least a dozen chemical attacks in Iraq and Syria, according to U.S. defense and intelligence officials:

Fox News reports: [An unnamed Pentagon] official said the base is attacked “fairly often” with conventional weapons, but Tuesday’s assault is believed to be the first chemical attack on U.S. forces in Iraq since they returned to the country in 2014. Nearly 5,000 U.S. troops are currently on the ground in Iraq and “hundreds” of those forces are located at the base, the official said.

Unlike the U.S. military, Dlovan Shukry Essa, a Kurdish fighter in northern Iraq charged with investigating possible chemical attacks, told Breitbart News that his force lacks the proper gear to safely carry out his duties.