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"I support the actions taken by the United States, the United Kingdom and France... This will reduce the regime's ability to further attack the people of Syria with chemical weapons," Secretary General Jens Stoltenberg said in a statement.

The Syrian regime's use of chemical weapons was "a clear breach of international norms and agreements", the statement said.

"NATO considers the use of chemical weapons as a threat to international peace and security, and believes that it is essential to protect the Chemical Weapons Convention," the statement added.

"This calls for a collective and effective response by the international community."

The airstrikes early on Saturday come a week after a suspected deadly gas attack on the rebel-held Damascus suburb of Douma that left dozens dead.

Loud explosions ripped through Syria’s capital and the sky turned orange as Syrian air defence units fired surface-to-air missiles in response to joint airstrikes by the United States, France and Britain.

Shortly after the one-hour attack ended, vehicles with loudspeakers roamed the streets of Damascus blaring nationalist songs.

"Good souls will not be humiliated," Syria's presidency tweeted after the airstrikes began. 

Immediately after the attack, Syrian state TV, broadcasting live from the landmark Omayyad Square, showed crowds of civilians mixing with men in uniform, including vehicles with flags. 

US President Donald Trump announced on Friday night that the three allies had launched military strikes to punish Syrian President Bashar Assad for alleged chemical weapons use and to prevent him from doing it again. 

Trump said Washington is prepared to "sustain" pressure on Assad until he ends what the president called a criminal pattern of killing his own people with internationally banned chemical weapons.

The Syrian government has repeatedly denied any use of banned weapons. 

Agencies contributed to this report.