The US has launched a military strike on a Syrian government target in retaliation for their chemical weapons attack on civilians earlier in the week, President Donald Trump told the media.

"Tonight (Thursday), I ordered a targeted military strike on the air field in Syria from where the chemical attack was launched," Trump told reporters at Mar-a-Lago. 

The US launched 59 cruise missiles into Syria in the first direct military attack by the country on President Bashar al-Assad's regime since the civil war began.

The strike came in response to a chemical weapons attack that struck a town in northern Syria, Efe news reported on Thursday.

The US fired the Tomahawk missiles at the Shayrat air base in the Syrian city of Homs, from which the aircraft that carried out the chemical strikes earlier this week took off.

Trump confirmed that he ordered the unilateral military action against the Syrian government in response to gas attack that killed nearly 100, including many children and injured several hundreds.

"It is in this vital national security interest of the US to prevent and deter the spread and use of deadly chemical weapons," he said.

The attack came days after reports of a chemical weapons attack emerged from the battleground in Syria, which the West said the Syrian government should be blamed for, Xinhua news agency reported.

Trump also called on "all civilised nations" to work to end the "slaughter and bloodshed" in Syria.

He added: "There can be no dispute that Syria used banned chemical weapons, violated its obligations under the Chemical Weapons Convention and ignored the urging of the UN Security Council." 

"Years of previous attempts at changing Assad's behaviour have all failed and failed very dramatically."

A US defence official said the strike was targeted on runway, aircraft and fuel points. The missiles were launched from warships in the Eastern Mediterranean, CNN reported.

Strikes were over "until another decision is made," the official said.

Just minutes before the missile attack, Russia warned the US of the "negative consequences" of military strikes on Syria.

"We have to think about negative consequences and all the responsibility if military action occurred will be on shoulders of those who initiated such doubtful and tragic enterprise," Russia's Deputy Ambassador to the UN, Vladimir Safronkov said, Efe news reported.

Members of the UN Security Council were still negotiating a resolution in response to the chemical attack.

The US Ambassador to the Council, Nikki Haley, had already warned in the previous meeting that Washington could initiate a unilateral strike on Syria if the UN still refrained from taking military action.

Thursday's attack marked Trump's first military order since his inauguration in January, as other US military operations in Syria, Yemen and Iraq were carried out under the authority of his commanders.