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".
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, warned that Western military strikes would risk starting a war.
Despite firm denials from the Assad government, there is an abundance of evidence that Syrian forces, with Russian connivance, have been using chemical weapons against their own people on a regular basis ever since.
Many Western politicians feel that - with all the moral grey areas of this situation - they cannot sell the pass on this issue yet again. It has become a test case for the international rule of law, which is under severe pressure on many fronts.