Washington - US Defense Secretary James Mattis said Friday that the airstrikes on targets in Syria were a "one-time shot" to send a strong message to Syrian President Bashar al-Assad. But he is not ruling out further attacks.

Mattis made the remarks at a briefing from the Pentagon Friday night, an hour after US President Donald Trump announced that the US and its allies France and Britain launched military strikes on Syria.

The defense secretary said the airstrikes were launched against several sites that he said helped provide Assad’s ability to create chemical weapons. He said the strikes have “gone to great lengths to avoid civilians and foreign casualties”.

"The purpose of our actions tonight is to establish a strong deterrent against the production, spread and use of chemical weapons," Trump said in an address to the nation.

Explosions were reported near the Syrian capital Damascus, and Syrian state television said government forces had shot down more than a dozen missiles, according to a BBC report.