Last week's U.S. strike on an Assad regime air base destroyed a fifth of regime leader Bashar al-Assad's working warplanes, Pentagon chief Jim Mattis said Monday.

"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 percent of Syria's operational aircraft," Mattis said in a statement.

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

Trump last week ordered missile strikes against an air field in Syria that U.S. intelligence believes was used to carry out an attack with the chemical agent sarin.