In a combined operation, the US, Britain and France today launched military strikes against Bashar al-Assad's Syrian regime, President Donald Trump announced, as he alleged the war-torn nation of using chemical weapons against its own people.

Trump said he has ordered "precision strikes" against Syria, where dozens of people were killed last weekend in a suspected toxic gas attack on Douma, the largest town in a former rebel stronghold outside Damascus.

Describing the strike as a deterrent, Trump said the US would maintain pressure on Syria until the Assad regime suspends use of chemical weapons.

"America does not seek an indefinite presence in Syria," he said, thanking the UK and France for joining the US in its fight against the Syrian regime.

"My fellow Americans, a short time ago, I ordered the United States armed forces to launch precision strikes on targets associated with the chemical weapons capabilities of Syrian dictator Bashar al-Assad. A combined operation with the armed forces of France and the United Kingdom is now underway. We thank them both," Trump said in his address to the nation.

"Today, the nations of Britain, France, and the United States of America have marshaled their righteous power against barbarism and brutality," Trump said.

"This massacre was a significant escalation in a pattern of chemical weapons use by that very terrible regime," Trump alleged.

"The evil and the despicable attack left mothers and fathers, infants and children, thrashing in pain and gasping for air. These are not the actions of a man; they are crimes of a monster instead," he said.

The US president sought to bring back the vestiges of World War I, after which civilised nations joined together to ban chemical warfare.

He noted that chemical weapons not only inflict gruesome suffering but also even small amounts of them can unleash widespread devastation.

"The purpose of our actions tonight is to establish a strong deterrent against the production, spread, and use of chemical weapons. Establishing this deterrent is a vital national security interest of the United States," Trump said.

"The combined American, British, and French response to these atrocities will integrate all instruments of our national power -- military, economic, and diplomatic. We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents," he said.

"The nations of the world can be judged by the friends they keep. No nation can succeed in the long run by promoting rogue states, brutal tyrants, and murderous dictators," he said.

Trump said Assad's recent attack was a fallout of Russia's "failure" to keep the promise of eliminating Syria's chemical weapons.

"In 2013, President Putin and his government promised the world that they would guarantee the elimination of Syria's chemical weapons. Assad's recent attack are the direct result of Russia's failure to keep that promise," Trump said.

"Russia must decide if it will continue down this dark path, or if it will join with civilised nations as a force for stability and peace. Hopefully, someday we'll get along with Russia, and maybe even Iran -- but maybe not," he said.