Defense Secretary James Mattis said Tuesday “there is no doubt” that Syrian President Bashar al-Assad’s regime is “responsible” for gassing its own people and warned that the U.S. would respond again if Assad were to launch another chemical strike.

“I have personally reviewed the intelligence and there is no doubt the Syrian regime is responsible for the decision to attack and for the attack itself,” Mattis said during an afternoon briefing at the Pentagon.

Mattis, speaking alongside U.S. Central Command head Gen. Joseph Votel, provided more details about why the Trump administration ordered U.S. warships to launch 59 Tomahawk cruise missiles at a Syrian air base last week.

Mattis said the U.S. began “a deliberate process led by the National Security Council to recommend diplomatic and military options to the president.”

“We met over several days and I spoke with some of our allies,” Mattis said. “The National Security Council considered the near century-old international prohibition against the use of chemical weapons."

Mattis maintains the Syrian regime repeatedly and deliberately violated the international law. The Pentagon says the response was a retaliatory strike for Assad’s chemical attack on innocent civilians including children.

“We determined that a measured military response could best deter the regime from doing this again,” he said. “As always, we examined how best to avoid civilian casualties in the execution of the strike and our actions were successful.”

The Pentagon said the strikes destroyed a fifth of Assad’s functioning, fixed-wing combat aircraft.

Mattis also warned if Assad continues to use chemical weapons, the United States would continue to respond -- though he did not say how.

He did say that America’s policy in Syria has not changed.

“Our priority remains the defeat of ISIS,” he said, adding that the international terror group represents a clear and present danger and an immediate threat to Europe and the United States.

Mattis warned, “The Syrian regime should think long and hard before it again acts so recklessly in violation of international law against the use of chemical weapons.”

While the U.S. focuses on defeating ISIS, other Trump administration officials including U.S. Ambassador to the United Nations Nikki Haley have made clear in recent days that they want the Assad regime gone in the end.