Trump administration open to additional strikes on Syria: White House

US President Donald Trump is open to authorising additional strikes on Syria if the use of chemical weapons continues in the country, the White House said on Monday.

"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," White House spokesman Sean Spicer said at a briefing on Monday.

Last Friday's attack damaged or destroyed 20 percent of Syria's operational aircraft, as well as fuel and ammunition sites and air defence capabilities, Defence Secretary Jim Mattis said 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," Mattis said in a statement.

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

US officials had earlier told Reuters that the US has made slight adjustments to its military activities in Syria to strengthen protection of American forces following the cruise missile strikes last week.

The officials, citing the need to safeguard operations in Syria, declined to specify what exact measures the United States has taken after the strikes, which Damascus, Tehran and Moscow have roundly condemned. They spoke on condition of anonymity.

Asked about the Reuters report, a US military spokesman later told a Pentagon news briefing that the US commander for the campaign has been "calling in the resources that he needs" to protect US forces in the wake of the strikes.

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The spokesman, Colonel John Thomas, also said US strikes in Syria had become more defensive and acknowledged the pace had slowed somewhat since last Friday.

"I don't think that is going to last for very long, but that is up to [Lieutenant General Stephen] Townsend," Thomas said, stressing there had been no attempts by Syria or its allies to retaliate against US troops so far.

President Donald Trump ordered the cruise missile strike on Syria's Shayrat air base last week in response to what Washington and its allies say was a poison gas attack by Syria's military in which scores of civilians died.

The chemical weapons attack killed at least 70 people, many of them children, in the Syrian town of Khan Sheikhoun. The Syrian government has denied it was behind the assault.

Moscow says there is no proof that the Syrian military carried out the attack, and called the US missile strike an act of aggression that violated international law.