Last week's US strike on a Syrian air base destroyed a fifth of President Bashar al-Assad's working warplanes, Pentagon chief James Mattis has said.

"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.

A Russian defence ministry spokesman said only six MiG-23s, plus a number of buildings, were destroyed and that only 23 of the missiles had reached Shayrat.

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 White House separately suggested that barrel bombs dropped by Assad's forces could be one of the military actions in Syria that could prompt a US response - if chemicals were involved.

"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."

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Barrel bombs - containers filled with explosives - are frequently used by government forces and making them a "red line" would significantly widen the criteria for retaliation.

The White House clarified later that Spicer was referring to barrel bombs carrying industrial chemicals such as chlorine.

"Nothing has changed in our posture," a senior administration official told the AFP news agency.

Government forces are suspected of using chlorine gas in attacks on several occasions throughout the war.

The US attack on Syria came after a suspected chemical attack had killed at least 87 people in Idlib's rebel-held Khan Sheikhoun last week, according to the UK-based Syrian Observatory for Human Rights.