US, British and French forces struck Syria with more than 100 missiles on Saturday in the first coordinated Western strikes against the Damascus government, targeting what they called chemical weapons sites in retaliation for a poison gas attack.

US President Donald Trump announced the military action from the White House, saying the three allies had “marshalled their righteous power against barbarism and brutality”. As he spoke, explosions rocked Damascus.

In response, the UN Secretary-General called for restraint and for countries to avoid any acts that could escalate the situation in Syria.

The UN Security Council was due to meet at 11am (1500 GMT) on Saturday at Russia’s request.

The bombing represents a major escalation putting the West in direct confrontation with Mr Assad’s superpower ally Russia, but is unlikely to alter the course of a multi-sided war which has killed at least half a million people in the past seven years.

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That in turn raises the question of where Western countries go from here, after a volley of strikes denounced by Damascus and Moscow as both reckless and pointless.

By morning, the Western countries said their bombing was over for now. Syria released video of President Bashar al-Assad, whose Russian- and Iranian-backed forces have already driven his enemies from Syria’s major towns and cities, arriving at work as usual, with the caption “morning of resilience”.

British Prime Minister Theresa May described the strike as “limited and targeted”. She said she had authorised the British action after intelligence indicated Mr Assad’s government was responsible for the attack using chemical weapons in the Damascus suburb of Douma a week ago.

"I believe that the action taken will have significantly degraded the Syrian regime's ability to use chemical weapons," she said.

"While the full assessment of the strike is ongoing, we are confident of its success," she added.

French President Emmanuel Macron said the strikes had been limited so far to Syria’s chemical weapons facilities.

His country's foreign minister, Jean-Yves Le Drian, said Saturday that the missile strikes had destroyed a "large part" of Damascus's stocks of chemical weapons.

"A large part of its chemical arsenal has been destroyed," Le Drian told BFM television. "A lot has been destroyed in last night's strikes."

With more than 100 missiles fired from ships and manned aircraft, the allies struck three of Syria’s main chemical weapons facilities, US Defense Secretary Jim Mattis and Joint Chiefs of Staff Chairman General Joseph Dunford said.

The targets included a Syrian centre in the greater Damascus area for the research, development, production and testing of chemical and biological weapons as well as a chemical weapons storage facility near the city of Homs.

A third target, also near Homs, contained both a chemical weapons equipment storage facility and a command post.

UN chief urges restraint

UN Secretary-General Antonio Guterres delayed a planned trip to Saudi Arabia to deal with the aftermath of the military action.

"I urge all member states to show restraint in these dangerous circumstances and to avoid any acts that could escalate the situation and worsen the suffering of the Syrian people," Mr Guterres said in a statement.

"Any use of chemical weapons is abhorrent. The suffering it causes is horrendous," Mr Guterres said.

The UN chief said it was important to act in line with the UN charter and international law.

He urged the UN Security Council to agree on establishing an inquiry that would identify the perpetrators of chemical attacks.

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Iran warned of "regional consequences" on Saturday following the strikes.

"The United States and its allies have no proof and, without even waiting for the Organisation for the Prohibition of Chemical Weapons to take a position, have carried out this military attack," said the foreign ministry in a statement.

Mr Mattis called the strikes a “one time shot”, although Mr Trump raised the prospect of further strikes if Mr Assad’s government again used chemical weapons.

“We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents,” the US president said in a televised address.

The Syrian conflict pits a complex myriad of parties against each other, with Russia and Iran giving Mr Assad military and political help that has largely proven decisive over the past three years in crushing any rebel threat to topple him. Fractured opposition forces have had varying levels of support from the West, Arab states and Turkey.

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The United States, Britain and France have all bombed the Islamic State group in Syria for years and had troops on the ground to fight them, but refrained from targeting Mr Assad’s government apart from a volley of US missiles last year.

Although the Western countries have all said for seven years that Mr Assad must leave power, they held back in the past from striking his government with no wider strategy to defeat him.

Syria says it 'absorbed' attack

Mr Assad’s government and allies responded outwardly with fury, although there were also clear suggestions that they considered the attack a one-off, unlikely to harm Mr Assad.

A senior official in a regional alliance that backs Damascus told Reuters the Syrian government and its allies had “absorbed” the attack. The sites that were targeted had been evacuated days ago thanks to a warning from Russia, the official said.

“If it is finished, and there is no second round, it will be considered limited,” the official said.

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Russia’s ambassador to the United States, Anatoly Antonov, said on Twitter: “Again, we are being threatened. We warned that such actions will not be left without consequences.”

Syrian state media called the attack a “flagrant violation of international law.” An official in Iran’s Revolutionary Guards said it would cause consequences that were against US interests.

French Defence Minister Florence Parly said the Russians “were warned beforehand” to avoid inadvertent escalation.

Syria claims to have shot down missiles

At least six loud explosions were heard in Damascus and smoke was seen rising over the city, a Reuters witness said. A second witness said the Barzah district of Damascus had been hit in the strikes. Barzah is the location of a major Syrian scientific research centre.

Iran’s Foreign Ministry condemned the US-led attacks and said Washington and its allies would bear responsibility for the consequences in the region and beyond, state media reported.

State-controlled Syrian TV said Syrian air defences shot down 13 missiles fired in the attack. The Russian defense ministry said none of the rockets launched had entered zones where Russian air defense systems are protecting military facilities in Tartus and Hmeimim.

The combined US, British and French assault appeared more intense than a similar strike Mr Trump ordered almost exactly a year ago against a Syrian air base in retaliation for an earlier chemical weapons attack that Washington attributed to Assad.

Mr Mattis said the United States conducted the air strikes with conclusive evidence that chlorine gas was used in the April 7 attack in Syria. Evidence that the nerve agent sarin also was used was inconclusive, he said.

Allegations of Mr Assad’s chlorine use are frequent in Syria’s conflict, raising questions about whether Washington had lowered the threshold for military action in Syria by deciding to strike after a chlorine attack. Syria agreed in 2013 to give up its chemical weapons. It is still permitted to have chlorine for civilian use, although its use as a weapon is banned.

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Mr Mattis, who US officials said had earlier warned in internal debates that too large an attack would risk confrontation with Russia, described the strikes as a one-off to dissuade Assad from “doing this again”.

But a US official familiar with the military planning said there could be more air strikes if the intelligence indicates Mr Assad has not stopped making, importing, storing or using chemical weapons including chlorine. The official said this could require a more sustained U.S. air and naval presence in the region, as well as more surveillance.

US says it has proof Syrian government responsible

The US State Department said the United States has proof at "a very high level of confidence" that the Syrian government carried out the attack but is still working to identify the mix of chemicals used.

"I'm not going to say which day we absolutely knew that there was proof. The attack took place on Saturday, we know for a fact that it was a chemical weapon," spokeswoman Heather Nauert said.

"We know that there are only certain countries like Syria that have delivery mechanisms and have those types of weapons."

Asked whether she could say that the United States has proof that Assad's regime was behind the strike, Nauert said: "Yes."

The White House said they had also received intelligence disputing Russia's claim that Britain had 'staged' the alleged chemical attack.

Russia says Britain staged Syria attack

The Russian military on Friday said it had proof that an alleged chemical weapons attack in Syria's Eastern Ghouta was staged on orders from London, a claim dismissed by Britain as "a blatant lie."

In an earlier press conference, Russian defence ministry spokesman Igor Konashenkov said Britain had pressured the White Helmets, who act as first responders in rebel-held areas, to fake the suspected chemical attack in the town of Douma.

The military has "proof that testifies to the direct participation of Britain in the organising of this provocation in Eastern Ghouta," said Mr Konashenkov.

The attack was staged on April 7, he argued, just as Syrian forces were battling the rebel group Jaish al-Islam, which was shelling Damascus from April 3 to 6.

Britain's UN Ambassador Karen Pierce called the spokesman's remarks "grotesque" and "a blatant lie."

"It's some of the worst piece of fake news we've seen yet from the Russia propaganda machine," Ms Pierce told reporters at UN headquarters in New York.

"I would like to take this opportunity to state categorically that... Britain has no involvement and would never have any involvement in the use of a chemical weapon."

Russia has repeatedly accused rebels of staging or spreading rumours of chemical attacks, but the involvement of Britain is a new claim.

It comes as London has blamed Moscow for a nerve agent attack on a former double agent and his daughter in the English city of Salisbury last month.

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'We know a chemical weapon was used'

Chemical weapons experts for the Organisation for the Prohibition of Chemical Weapons (OPCW) has arrived in Syria to investigate the suspected poison gas attack.

The investigators, who are only mandated to determine if chemical weapons were used and not who used them, were expected to start their investigations into the Douma incident on Saturday, the Netherlands-based organisation said.

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- Reporting by Reuters, AFP and AAP