The strikes hit a military airport and related targets, killing six.
A joint command centre made up of the forces of Russia, Iran and militias supporting Syrian President Bashar al-Assad on Sunday said the USA strike on a Syrian air base on Friday crossed "red lines" and it would respond to any new aggression and increase its support for its ally.
The strikes were in reaction to what Washington says was a poison gas attack by the government of Syrian President Bashar al-Assad that killed at least 70 people in rebel-held territory. "We are prepared to do more".
In a letter to Congress on Saturday, Trump said his aim was to "degrade the Syrian military's ability to conduct further chemical weapons attacks and to dissuade the Syrian regime from using or proliferating chemical weapons, thereby promoting the stability of the region and averting a worsening of the region's current humanitarian catastrophe".
Rights monitors warned, however, that Syrian jets were already flying sorties from the base again as night fell Friday.
The strikes against Shayrat Airfield in Homs Governorate were ordered by US President Donald Trump.
Assad's office called the strike "foolish and irresponsible", and Moscow announced a series of retaliatory steps including plans to strengthen Syrian air defenses. And Trump, who has attempted to enact a ban on Syrians and those in five other majority-Muslim nations from entering the United States, has not indicated that he is willing to accept more Syrians who are fleeing violence. "Those days are over".
Iran, which has provided crucial military and political support to Assad, meanwhile called for a fact-finding mission to determine what caused the chemical attack in Khan Sheikhoun.
It was Trump's biggest foreign policy decision since taking office in January and the kind of direct intervention in Syria's six-year-old civil war his predecessor Barack Obama avoided.
Russia's envoy to the United Nations slammed the USA action, calling it "a flagrant violation of worldwide law and an act of aggression" in remarks at a UN Security Council meeting on Friday.
In an interview on CBS News's "Face the Nation" set to air Sunday, Secretary of State Rex Tillerson said the administration's top priority is defeating the Islamic State. A fully developed proposal is expected to be delivered to Trump's desk in the near future, a senior administration official said.
"The use of chemical weapons against innocent people will not be tolerated".
"I find it very disappointing but - sadly, I have to tell you - not all that surprising".
A spokesman for President Recep Tayyip Erdogan of Turkey, Ibrahim Kalin, said the American strikes had been a positive response to "war crimes" in Syria, where the six-year civil war has led to almost 400,000 deaths and created a refugee crisis as millions sought to flee.
"We're hopeful that we can prevent a continuation of the civil war and that we can bring the parties to the table to begin the process of political discussions" between the Assad government and various rebel groups, he said.
UN Secretary General Antonio Guterres said all parties must remember their "shared duty to uphold global standards of humanity".
Britain, Canada, France, Germany, Israel, Japan, Saudi Arabia and Turkey all supported Washington, with Ankara also calling for a no-fly zone in Syria. Obama aides said at the time that the president wanted broad political and public support before acting after years of USA military conflicts in the Middle East and Central Asia.
Russia's Defense Ministry responded to the attack by calling in the US military attache in Moscow to say that at midnight Moscow time (5 p.m. EDT) it would close down a communications line used to avoid accidental clashes between Russian and USA forces in Syria, Interfax new agency said.
"Tonight I call on all civilized nations to join us in seeking to end this slaughter and bloodshed in Syria and also to end terrorism of all kinds and all types", he said.
The missiles targeted aircraft, hardened aircraft shelters, petroleum and logistical storage, ammunition supply bunkers, air defence systems, and radars, according to the US Department of Defense (DoD). Russian Federation had its own military personnel at the Syrian military airport that the USA struck with cruise missiles.
"We do not have any information that suggests that Russian Federation was part of the military attack undertaken using the chemical weapons", Tillerson said.
It destroyed six planes under fix and several buildings, including a storage depot and radio station, it said.
A Syrian military source said its armed forces had also received advance warning. Canada condemns all uses of chemical weapons.
In the blighted town itself, residents mourning their dead welcomed the U.S. strike as a way to pressure Damascus. He did not rule out additional strikes if Assad continued to engage in atrocities against rebel forces with either chemical or conventional weapons.