The U.S. missile strike on Syria’s Sharyat airfield Thursday took out 20% of President Bashar Assad’s operational warplane fleet, according to a statement issued by Pentagon chief Jim Mattis on Monday.

In the statement, Mattis described the volley of 59 Tomahawk missiles as a “measured response” to Assad’s “use of chemical weapons.” He also said that the Syrian government would be “ill-advised ever again to use chemical weapons.

“The assessment of the Department of Defense is that the strike resulted in the damage or destruction of fuel and ammunition sites, air defense capabilities, and 20% of Syria’s operational aircraft,” Mattis said in the statement.

“The Syrian government has lost the ability to refuel or re-arm aircraft at Shayrat airfield and at this point, use of the runway is of idle military interest,” he added.

The release follows earlier comments issued by the U.S. military’s Central Command spokesperson Colonel John Thomas, who said that more than 20 Syrian jets had been destroyed at Sharyat airfield, near the city of Homs, Agence France Presse reports. According to Thomas, the U.S. had tried to avoid hitting runways in order to convey that the military action was a reaction to the suspected chemical attack rather than indicative of future involvement in Syria’s civil war.

The Syrian military had reported significant damage but Russian defense said only six Syrian warplanes were destroyed, according to the BBC.

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Write to Joseph Hincks at joseph.hincks@time.com.