Last week's US strike on a Syrian air base destroyed a fifth of President 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.