Jim Mattis called Friday's strike a 'measured response' to the regime's 'use of chemical weapons'.
Washington: 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 on Monday.
"The assessment of the Department of Defence is that the strike resulted in the damage or destruction of fuel and ammunition sites, air defence capabilities, and 20 percent of Syria's operational aircraft," Mattis said in a 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.
Earlier, the US military's Central Command spokesman Colonel John Thomas said the US strike at Shayrat airfield near Homs in central Syria had destroyed more than 20 Syrian jets.
Mattis called Friday's strike a "measured response" to the regime's "use of chemical weapons."
"The Syrian government would be ill-advised ever again to use chemical weapons," Mattis said.
Friday's strike saw two US destroyers unleash a volley of 59 Tomahawk missiles at the airbase.
Thomas said the runways were deliberately avoided because the United States was trying to draw a clear line that its military action was in response to the suspected chemical attack, and not a signal of willingness to get more involved in Syria's brutal civil war.