The UK has deployed four RAF Tornados in a “limited and targeted strike” against Syria.
The US and France have also launched co-ordinated strikes in response to an alleged chemical attack on the Syrian town of Douma last week.
Prime Minister Theresa May said there was “no practicable alternative to the use of force” to deter the use of chemical weapons by the Syrian regime.
Syrian state media called the strikes a “violation of international law”.
Several large explosions have been heard in the Syrian capital Damascus.
In a statement Mrs May said that the Syrian regime had demonstrated a “persistent pattern of behaviour” when it came to the use of chemical weapons, that “must be stopped”.
“We have sought to use every possible diplomatic channel to achieve this,” the prime minister said.
“This is not about intervening in a civil war. It is not about regime change.
“It is about a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties.”
Foreign Secretary Boris Johnson welcomed the news in a tweet, saying the “world is united in its disgust for any use of chemical weapons, but especially against civilians”.
He said the military strikes were targeting “major chemical weapons facilities in Syria”.
What is an RAF Tornado?
The Tornado has been one of the mainstays of the RAF since first entering service in 1980 and the aircraft were used to enforce no-fly zones in Iraq.
It is mainly used as a strike or attack aircraft.
Weapons such as the Storm Shadow cruise missile mean that the Tornado can hit targets from a significant distance. The MoD describes the missile as being designed for “long range, highly accurate, deep penetration” against enemy command and control bunkers. It is fired from a Tornado GR4.
Tornado GR4s are also equipped with Brimstone missiles, an effective anti-armour weapon and can also be used for all-weather, day and night tactical reconnaissance.
‘Chemical weapon stockpile’
The Ministry of Defence (MoD) said that the four RAF Tornados launched Storm Shadow cruise missiles at a Syrian military facility.
The former missile base was assessed to have been used by the Syrian regime to “keep chemical weapon precursors stockpiled in breach of Syria’s obligations under the Chemical Weapons Convention,” the MoD said in a statement.
Defence Secretary Gavin Williamson added: “The international community has responded decisively with legal and proportionate military force. Let these united actions send a clear message to the regime – the use of chemical weapons is categorically unacceptable and you will be held to account.”
Syrian state media said the air strikes were “a flagrant violation of international law”.
The official Sana news agency cited an unnamed source as saying: “When terrorists failed, the USA, France and Britain intervened and committed aggression against Syria.
“The American, French and British aggression against Syria will fail.”
Scotland’s First Minister Nicola Sturgeon said Mrs May had not answered how the action, “taken without parliamentary approval, will halt their [Syria’s] use” of chemical weapons “or bring long term peace”.