The top U.S. general said Thursday that an Islamic State rocket that hit a military base used by hundreds of U.S. troops in northern Iraq contained chemical agents that cause human skin to blister.

Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, was asked at a Senate Armed Services Committee hearing Thursday about the disclosure a day earlier that the Islamic State may have attacked Qayara West air base with chemical weapons.

U.S. officials had said Wednesday that an oily substance found on a fragment of the rocket that landed inside the security perimeter of the base initially tested positive for mustard agent, but that a second test was negative. They said further laboratory testing was scheduled.

"We assess it to be a sulfur-mustard blister agent," Dunford said, adding that no one was injured by it.

He called the incident a "concerning development."

Dunford made no mention of the U.S. presence on the Qayara West base, but an official on Wednesday said "hundreds" of U.S. troops are there helping the Iraq security forces prepare for a coming offensive in Mosul.

Dunford said that while the Islamic State has a "chemical warfare network," it has only limited means of making effective chemical weapons. He noted that last week the U.S. military attacked a former pharmaceutical plant near Mosul in northern Iraq that U.S. officials said the Islamic State was using to produce mustard agent and other chemicals for military use.