The surprise US strike drew quick reactions from the friends and foes of the Syrian government and further strained relations with Russia, a key ally of Assad, which suspended a deal with the US to prevent midair collisions in Syria's crowded skies.
Saudi Arabia and Turkey, which support the Syrian opposition, welcomed the missile strike, with Riyadh calling it a "courageous decision" by Trump. Iran, another close ally of the Syrian government, condemned the strike, describing "unilateral action" as "dangerous."
The bombing represents Trump's most dramatic military order since taking office and thrusts the US administration deeper into the complex Syrian conflict.
The Obama administration threatened to attack Assad's forces after previous chemical attacks, but never followed through. Trump called on "all civilised nations" to join the US in seeking an end to the carnage in Syria.
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About 60 US Tomahawk missiles hit the Shayrat air base, southeast of Homs, a small installation with two runways, where aircraft often take off to bomb targets in northern and central Syria. The US missiles hit at 3:45 am (local time) today morning and targeted the base's airstrips, hangars, control tower and ammunition areas, US officials said.
The Syrian military said at least seven people were killed and nine wounded in the missile strike. A Syrian opposition monitor put the death toll at four, including a general and three soldiers.
The Kremlin said President Vladimir Putin believes the US strike is an "aggression against a sovereign state in violation of international law."
Putin spokesman Dmitry Peskov said in statement carried by today Russian news agencies that Putin believes the US launched the strikes under a "far-fetched pretext."
Russia's Foreign Ministry later said it is suspending a memorandum with Washington signed after Russia began an air campaign in support of Assad in September 2015 under which the two countries exchange information about sorties over Syria.
A US-led coalition has been bombing Islamic State targets in Syria since 2014, while Russia's air force has been striking both extremist groups and Syrian rebels in order to aid Assad's forces.
A Syrian opposition group, the Syrian Coalition, welcomed the US attack, saying it puts an end to an age of "impunity" and should herald the start of a larger campaign against Damascus.
Maj Jamil al-Saleh, a US-backed rebel commander based in the area where the US attack took place, told The Associated Press he hoped the strike would be a "turning point" in the six-year-old war, which has killed an estimated 400,000 people.
Syria rejected the accusations, and blames opposition fighters for stockpiling the chemicals. Russia has said the toxic agents were released when a Syrian airstrike hit a rebel chemical weapons arsenal on the eastern outskirts of Khan Sheikhoun, and that blame should not be apportioned until a full investigation has been carried out.
Russia's intervention in Syria since September 2015 has turned the balance of power in Assad's favor, and Moscow has used its veto power at the Security Council on several occasions to prevent sanctions against Damascus.
A survivor of the chemical attack told the AP he hopes the US missile attack puts an end to government airstrikes, creating a safe area for civilians.
The US had initially focused on diplomatic efforts after the chemical attack, and had pressed the UN Security Council to adopt a resolution drafted with Britain and France that would have condemned Syria's suspected use of chemical weapons. But the vote was cancelled because of differences among the 15 members.