US president Donald Trump has said air strikes against Syria are "underway" in a combined operation with the UK and France.

Four Royal Air Force Tornados have contributed to strikes in Syria, launching Storm Shadow missiles at a former missile base 15 miles west of Homs in a "successful attack", the Ministry of Defence said.

Giving a statement at the White House, Mr Trump said the strikes were in response to the alleged chemical weapons attack in the town of Douma last Saturday.

He said: "This evil and despicable act left mothers and fathers and children thrashing in pain and gasping for air.

READ MORE: 'No alternative to use of force against Assad'

"The combined American, British and French response will integrate all instruments of our national power."

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.

Adding that "every possible diplomatic channel" had been sought before the decision to launch targeted strikes had been taken, she said: "This persistent pattern of behaviour must be stopped - not just to protect innocent people in Syria from the horrific deaths and casualties caused by chemical weapons but also because we cannot allow the erosion of the international norm that prevents the use of these weapons.

WATCH: Theresa May announces military action against Syria

"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."

Mrs May said the action would also send a "clear signal" to anyone else who believed they could use chemical weapons "with impunity".

She said: "This is the first time as Prime Minister that I have had to take the decision to commit our armed forces in combat - and it is not a decision I have taken lightly.

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"I have done so because I judge this action to be in Britain's national interest.

"We cannot allow the use of chemical weapons to become normalised - within Syria, on the streets of the UK, or anywhere else in our world."

French president Emmanuel Macron said there was no doubt the Syrian regime was responsible for the chemical attack in Douma.

He said: "We cannot tolerate the trivialisation of the use of chemical weapons, which represent an immediate danger for the Syrian people and for our collective security.

WATCH: Theresa May announces military action against Syria

"The red line set by France in May 2017 has been crossed."

Air strikes by the US, UK and France were launched at 9pm EST (2am BST) and destroyed important infrastructure at three sites connected to the Syrian regime's chemical weapons programme.

General Joseph Dunford, chairman of the US Joint Chiefs of Staff, said the first was at a scientific research centre in greater Damascus, involved in the development and production of chemical warfare.

The second was at a chemical weapons storage facility west of Homs, while the third was at a chemical equipment storage facility and important command post.

READ MORE: 'No alternative to use of force against Assad'

He added: "Important infrastructure was destroyed which will result in a set-back for the Syrian regime. They will lose years of research and development, storage and equipment."

Four Royal Air Force Tornados have contributed to strikes in Syria, launching Storm Shadow missiles at a former missile base 15 miles west of Homs in a "successful attack", the Ministry of Defence said.

The MoD said the regime was "assessed to keep chemical weapon precursors stockpiled in breach of Syria's obligations under the Chemical Weapons Convention" at the site.

Defence Secretary Gavin Williamson said: "The reprehensible use of chemical weapons in Douma is further evidence of the Syrian regime's appalling cruelty against its own people.

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"We will not stand by whilst innocent civilians, including women and children, are killed and made to suffer.

"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."

US Defence Secretary James Mattis and General Joe Dunford told reporters at a Pentagon briefing that the strikes were over and there had been no reports of any allied losses.

General Dunford described the operation as a "one time shot".

He said the only resistance was some Syrian surface-to-air missile activity.

WATCH: Theresa May announces military action against Syria

General Dunford said there was no co-ordination with the Russians and "nor did we notify them".

Mr Mattis said double the number of weapons were used in the strikes compared with similar attacks on Syria carried out last year.