Defense Secretary James Mattis on Monday warned Syrian President Bashar Assad not to use chemical weapons again, saying the U.S. "will not passively stand by."
"The U.S. military strike against Shayrat airfield on April 6 was a measured response to the Syrian government's use of chemical weapons," Mattis said in a statement.
"President Trump directed this action to deter future use of chemical weapons and to show the United States will not passively stand by while Assad murders innocent people with chemical weapons, which are prohibited by international law and which were declared destroyed," he added.
"The Syrian government would be ill-advised ever again to use chemical weapons."
Trump last Thursday ordered a missile strike on a Syrian air base in response to a chemical attack in the country that left at least 70 civilians dead, including children. U.S. and other Western officials said Assad's forces used a nerve gas.
U.S. Central Command spokesman Col. John Thomas said Monday that the U.S.'s 59 Tomahawk missiles destroyed more than 20 Syrian aircraft, aircraft shelters, fuel storage, ammunition supply bunkers, radars and air defense systems. The Pentagon deliberately avoided destroying suspected chemical weapon stockpiles as to not inadvertently harm civilians.
Many have questioned the decision not to strike the base’s runways, but Thomas said they were avoided because the United States was trying to “send a message” instead of increasing its involvement in Syria's civil war.
“The reason for this strike was limited to and very specifically directed at the president ordering us to take action against the use of chemical weapons..from that airbase,” Thomas told reporters. “It has not been a position of Centcom or the United States to take action in the, what you might call civil war, going on with Assad in Syria.”
The avoidance of the runways has become more of an issue since last Friday, when reports surfaced that Syrian warplanes had carried out attacks from the installation, leading some to question the efficacy of the U.S. response.
Trump pushed back against the reports and took to Twitter over the weekend to defend the strike.
"The reason you don't generally hit runways is that they are easy and inexpensive to quickly fix (fill in and top)!" Trump wrote on Twitter Saturday.
Mattis, meanwhile, said on Monday that the Syrian government has lost the ability to refuel or rearm aircraft at the airfield "and at this point, use of the runway is of idle military interest."
Responding to a reporter’s question on whether Assad's forces still could launch chemical attacks from the base, Thomas replied, “I don’t know. You’d have to draw a lot of conclusions there. I know they are deprived of more than 20 of their aircraft and that they don’t have much fuel on site. It’s not easy to replace fuel. It creates a logistical problem.”
"You’re not going to see a lot of sustained coordinated operations out of that airfield until they spend a lot of time rehabilitating the base,” he added.
Updated at 3:11 p.m.