“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 on Monday.

“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 government’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 US 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 civil war. Barrel bombs

The US said barrel bombs of Assad’s forces could be one of the military actions in Syria that could potentially prompt a US response.

“When you watch babies and children being gassed and suffer under barrel bombs, you are instantaneously moved to action,” White House spokesman Sean Spicer told a news briefing.

“I think this president has made it very clear that if those actions were to continue, further action will definitely be considered by the United States.”

READ MORE: Syria’s civil war explained from the beginning

Adding barrel bombs to the list of forbidden weapons significantly widens the sort of military activity by Assad that would warrant action.

“The sight of people being gassed and blown away by barrel bombs ensures that if we see this kind of action again, we hold open the possibility of future action,” he said.

“If you gas a baby, if you put a barrel bomb in to innocent people, I think you will see a response from this president,” he added. DM