WASHINGTON — U.S. Defense Secretary Ash Carter and lawmakers expressed concern that Islamic State is moving to develop chemical weapons even as the U.S.-led coalition makes progress against the terrorist group in Iraq.

The coalition is "going after ISIL's attempts to develop chemical weapons, as we continue to ensure that U.S., coalition and Iraqi troops are vigilantly protected from that threat," Carter said Thursday at a Senate Armed Services Committee hearing, using an acronym for the terrorist group.

In one of the "single largest airstrikes" of the campaign against the Islamic State, Carter said, warplanes last week destroyed a pharmaceutical facility near Mosul, Iraq, that the group planned to use to make chemical weapons. Pentagon officials have said the plant was intended to produce chlorine gas.

On Tuesday, a rocket that may have carried a chemical agent landed near the security perimeter at the Qayyarah airbase in Iraq, where hundreds of U.S. personnel are stationed to support the planned Iraqi assault to retake Mosul. Tests on the rocket were continuing.

"We assess it to be a sulfur mustard blister agent," Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, said at the hearing. "It went on one of our bases," where there's effective detection and protection equipment as well as decontamination capabilities, according to the general. He said the coalition has carried out 30 strikes over the past year targeting the Islamic State's "emerging chemical capability."

Carter said the coalition is making progress in countering Islamic State, with the aim of moving on the militants' strongholds in Mosul and Raqqa, Syria. The "final assault" on Mosul will begin when Iraq's Prime Minister Haider al-Abadi gives the order, Carter said.

Committee Chairman Sen. John McCain, an Arizona Republican who has long portrayed President Barack Obama as too timid in taking military action, said the president has "at least begun to unleash" U.S. forces against the Islamic State in Iraq.

But McCain said the administration "still has no plausible vision" for ending the conflict, and he dismissed Secretary of State John Kerry's efforts to negotiate with Russia for a cease-fire in the civil war that's devastating Syria.

"President Obama sent his intrepid but delusional secretary of state to tilt yet again at the windmill of cooperating" with Russian President Vladimir Putin, McCain said.

"Is there a Plan B here? Or do we just keep going back to the five-star hotels in Geneva" for Kerry's meetings with Russian Foreign Secretary Sergey Lavrov? McCain asked.