US President Donald Trump on Twitter has praised Western air strikes against the Syrian Government as "perfectly executed", and added "Mission Accomplished".

Key points: US, UK and France hit three chemical weapons sites in Syria

US, UK and France hit three chemical weapons sites in Syria US Defence Secretary says strikes were a "one-time shot"

US Defence Secretary says strikes were a "one-time shot" Strikes biggest intervention yet by Western powers against Assad regime

"A perfectly executed strike last night. Thank you to France and the United Kingdom for their wisdom and the power of their fine Military. Could not have had a better result. Mission Accomplished!" Mr Trump said in a Twitter post.

In 2003, then-President George W Bush spoke under a "Mission Accomplished" banner when he went aboard an aircraft carrier in California to declare that major combat operations in Iraq were over — just six weeks after the invasion.

The war dragged on for years, and Mr Bush was heavily criticised for his statement.

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On Saturday US, British and French forces pounded chemical weapons sites in Syria with air strikes in response to an alleged poison gas attack that killed dozens in the rebel-held town of Douma last week.

In a televised address to the nation, Mr Trump said the three nations had "marshalled their righteous power against barbarism and brutality".

The strikes were the biggest intervention by Western powers against President Bashar al-Assad in the country's seven-year-old civil war, which has pitted the US and its allies against Russia.

'Precise, overwhelming and effective'

The Pentagon said the strikes targeted a research centre in Damascus, along with a chemical weapons storage facility and command post west of Homs.

They said that the strikes had successfully hit every target and were aimed to deliver an unambiguous signal to the Syrian Government and deter the future use of chemical weapons.

The strikes significantly crippled Syrian President Bashar al-Assad's ability to produce chemical weapons, officials told reporters at a briefing, and the Pentagon was not aware of any civilian casualties resulting from the strikes.

Lieutenant General Kenneth F. McKenzie said the strikes were precise, overwhelming and effective.

Though some of Syria's chemical weapons infrastructure was still left, "I think we've dealt them a severe blow," General McKenzie said, adding it would set the program back years.

Sorry, this video has expired Trump announces 'precision strikes' against Syrian regime

Despite severely damaging the infrastructure with the strikes, General McKenzie said the Pentagon would not rule out the possibility of the Assad government still having the capability to use such weapons again.

"I would say there's still a residual element of the Syrian program that's out there," he said.

"I'm not going to say that they're going to be unable to continue to conduct a chemical attack in the future. I suspect, however, they'll think long and hard about it."

Russia's Ambassador to the US Anatoly Antonov said in a statement that Russia had warned that "such actions will not be left without consequences".

US Defence Secretary James Mattis said the strikes were a "one-time shot" to send a clear message to Mr Assad and his "murderous lieutenants".

"Clearly the Assad regime did not get the message last year," Mr Mattis said.

"We did everything we could in our intelligence assessment and our planning to minimise to the maximum degree … any chance of civilian casualties."

He said the US did not co-ordinate with Russia on the strike nor did it pre-notify the Russians.

"We specifically identified these targets to mitigate the risk of Russian forces being involved," Mr Mattis said.

However, a senior official in a regional alliance that backs Damascus said the sites that were targeted had been evacuated days ago thanks to a warning from Russia.

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

Sorry, this video has expired Syrian regime claims to have shot down US-led coalition missiles

Mr Trump said the strikes were in response to the "evil and the despicable" chemical attack on April 7 which "left mothers and fathers, infants and children, thrashing in pain and gasping for air".

"These are not the actions of a man, they are crimes of a monster," Mr Trump said, referring to Mr Assad.

UN secretary-general Antonio Guterres urged all nations to "show restraint in these dangerous circumstances".

"I have repeatedly expressed my deep disappointment that the Security Council failed to agree on a dedicated mechanism for effective accountability for the use of chemical weapons in Syria," Mr Guterres said.

"I urge the Security Council to assume its responsibilities and fill this gap."

British Prime Minister Theresa May said the strikes were not about intervening in a civil war nor were they about a regime change.

"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," Ms May said, referencing the recent nerve gas attack on a Russian double agent in England.

"It is a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties," she said.

Sorry, this video has expired Night vision shows the launch of anti-air missiles from Damascus after US-led strikes.

French President Emmanuel Macron said a "red line has been crossed".

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

Australian Prime Minister Malcolm Turnbull said Australia supported the strikes, which demonstrated a "calibrated, proportionate and targeted response".

"The use of chemical weapons by anyone, anywhere, under any circumstances is illegal and utterly reprehensible," Mr Turnbull said.

"The Assad regime must not be allowed to commit such crimes with impunity."

German Chancellor Angela Merkel also backed the air strikes, calling them a "necessary and appropriate" action to warn Syria against further use of chemical weapons.

And the European Union called for Russia and Iran to help stop more chemical weapons attacks by the Syrian Government, warning of fresh economic sanctions.

Sorry, this video has expired US Defence Secretary confirms strikes on Syria

The Pentagon said the US and its allies fired more than 100 missiles from ships and aircraft.

It could not confirm how many missiles hit their targets.

A pro-Assad official said about 30 missiles were fired at Syria and that about a third were shot down.

Several explosions were heard in Damascus. Black plumes of smoke were seen rising from in the east, and the sky turned orange.

After the attack finished vehicles with loudspeakers roamed the capital's streets blaring nationalist songs.

In a tweet, Syrian opposition leader Nasra al-Hariri called for an end to what he described as all attacks against civilians by the Syrian state and its allies, whether using chemical or conventional weapons.

"Maybe the regime will not use chemical weapons again, but it will not hesitate to use weapons the international community has allowed it, such as barrel bombs and cluster bombs," Mr Hariri said.

Sorry, this video has expired UK says chemical attack could only have been carried out by regime

Mr Trump said the US was not seeking an "indefinite presence" in Syria.

He said the US would be a "partner and a friend" to the Middle East but the fate of the region lay in the hands of its own people.

"As other nations step up their contributions, we look forward to the day when we can bring our warriors home," he said.

"No amount of American blood or treasure can produce lasting peace and security in the Middle East.

"We will try to make it better, but it is a troubled place."

Russia's Defence Ministry said the majority of missiles fired during the attack were intercepted by Syrian air defence systems using Soviet-produced hardware, including the Buk missile system.

Sorry, this video has expired Video purports to show remains of missile dropped on Douma

Sorry, this video has expired Syrian regime supporters dance in Damascus in celebration of the regime's response to the attacks.

ABC/wires