WASHINGTON — Future use of chemical weapons in Syria will trigger a strong response from the United States, Secretary of Defense Jim Mattis said today — but just what constitutes a "chemical weapon" is unclear.

Mattis, making his first appearance in the Pentagon briefing room since taking the top job at the Department of Defense, offered strong words against the decision by the Assad government to use chemical weapons in an April 4 attack, saying there is "no doubt the Syrian regime is responsible for the decision to attack and for the attack itself."

"I trust [Assad] regrets it now, considering the damage done to his air force, but he should think long and hard" before using such chemical weapons again, Mattis said.

In January 2016, the Organisation for the Prohibition of Chemical Weapons (OPCW) declared that all of Assad’s chemical weapons stockpile had been destroyed. But the U.S. has concluded the April 4 attack on the Syrian town of Khan Sheikhoun involved the use of a nerve agent similar to sarin gas.

In response, the U.S. launched a series of Tomahawk missiles at the airfield from which the Syrian planes used in that bombing raid had taken off, a move officials have described as a proportional response designed to deter the regime from using chemical weapons again in the future.

Asked about previous uses of chlorine by the Assad government, Mattis acknowledged that has happened, but stressed that "this time it was not chlorine, quite clearly, and we know that for certain. There is no doubt. This is a medical fact."

Pressed if a barrel bomb attack involving chlorine constitutes a chemical weapon attack, the secretary dodged, "I just want to say very clearly that the use of chemical weapons — contrary to the Genera Convention that Syria signed up for, using chemical weapons that Syria agreed under U.N. pressure to remove from their arsenal, the chemical weapons the Russians certified were gone — that if they use chemical weapons they are going to pay a very, very stiff price."

Then, when asked after the presser to clarify how chlorine fits into the definition of chemical weapons, Mattis told reporters that "I really don’t want to" clear up the issue.

Speaking on CNN after the press conference, Ret. Gen. Michael Hayden, the former head of both the National Security Agency and the Central Intelligence Agency, hinted that Mattis was leaning into that uncertainty in order to avoid setting a hard line that could lead to more military action.

"There is some ambiguity on the battlefield about chlorine," Hayden said. "At the the political level there would have to be significant casualties that would trigger this kind of response for the use of chlorine in the future."

And indeed, Mattis made efforts to stress that these strikes were not part of a broader change in U.S. posture in Syria.

In the days since the U.S. strikes, members of the Trump administration — most notably Secretary of State Rex Tillerson and U.N. Ambassador Nikki Haley — have ratcheted up their tone on Syria. But Mattis stressed that these strikes were targeted specifically as a response to the use of chemical weapons, and were not a precursor to a full-scale U.S. invasion against the Assad regime.

"What we have to look at here is a policy decision by the United States," Mattis said. "There is a limit I think to what we can do, and when you look at what happened with this chemical attack, we knew we could not stand passive on this. But it was not a statement that we could enter full-fledged, full-bore into the most complex civil war probably raging on the planet at this time."

The secretary also clarified a statement, released Monday, that 20 percent of the Syrian air force’s operational aircraft were destroyed in the Tomahawk strike, instead using a figure that around 20 aircraft were destroyed. That figure is in line with what DoD officials told reporters on Friday.

"I thought it was about 20 percent. I think it’s around 20 aircraft were taken out, which probably equates to about that, although I probably shouldn’t have used the 20 percent," Mattis said. "It’s around 20 aircraft, and that damage to the Syrian air force is pretty severe."

He also noted that the Syrian air force is "not in good shape. It’s been worn down by years of combat, plus significant maintenance problems."