U.S. Defense Secretary James Mattis give a news conference with Britain's Defence Secretary Michael Fallon (not shown) at Lancaster House in London March 31, 2017. Pool/Matt Dunham via Reuters Defense Secretary Jim Mattis says the Syrian regime would be "ill-advised" to use chemical weapons in the future, foreshadowing a potentially greater US response than the cruise missile strike that was carried out last week.

Mattis said in a statement that the strike, which consisted of 59 Tomahawk cruise missiles targeting the Shayrat air field on April 6, was a "measured response" to the Assad regime's use of chemical weapons.

In a break from his predecessor, President Donald Trump quickly authorized strikes against the Syrian government —a first for the US. According to Mattis, it was meant to deter future chemical weapons use, while showing the world that the US would "not passively stand by" when such atrocities are carried out.

Secretary of State Rex Tillerson said there could be "no doubt" that Assad's troops carried out the attack, and autopsies have showed that sarin gas was used. The Syrian government and Russia, its ally, have vigorously denied responsibility.

Russia said that instead, the Syrian air force perhaps carried out a conventional attack that hit a chemical weapons cache controlled by the rebels. However, as chemical weapons expert Dan Kaszeta explained to Bellingcat, sarin in storage consists of unmixed components, and dropping a bomb on them would not turn them into a nerve agent.

"It is an infantile argument," he wrote.

Meanwhile, Mattis' statement revealed some details of a damage assessment at the air field. It said the Tomahawk strikes destroyed or damaged fuel and ammunition sites, air defenses, and 20% of Syria's operational aircraft.

"The Syrian government would be ill-advised ever again to use chemical weapons," Mattis concluded.