President Donald Trump says he has approved US military strikes against Syrian chemical weapons sites, in collaboration with the UK and France.
The strikes are in response to an alleged chemical attack on the Syrian town of Douma last week.
"A combined operation with the armed forces of France and the United Kingdom is now underway," President Trump said in an address to the nation.
Explosions were reported near the Syrian capital Damascus.
UK Prime Minister Theresa May confirmed British involvement, saying there was "no practicable alternative to the use of force".
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But she also said the strikes were not about "regime change".
The strikes were ordered "on targets associated with the chemical weapons capabilities" of the Syrian government, Mr Trump said.
The US president said the purpose was "to establish a strong deterrent against the production, spread and use of chemical weapons".
"These are not the actions of a man, they are the crimes of a monster instead," he said of Syrian President Bashar al-Assad.
Syria has denied carrying out the attack and its ally, Russia, had warned that Western military strikes would risk starting a war.
A US official told Reuters news agency that Tomahawk cruise missiles were being used against multiple locations in Syria.
The agency also quoted a witness in Damascus as saying "at least six loud explosions" were heard in the capital.
Syrian state television also confirmed strikes on Damascus. The country's air defences have also been deployed, reports say.
British-based monitoring group the Syrian Observatory for Human Rights said strikes had hit the Syrian Scientific Research Facility in the capital, along with several military sites.
