(AFP) - The United States, Britain and France carried out a wave of punitive strikes against Bashar al-Assad's Syrian regime on Saturday in response to alleged chemical weapons attacks that President Donald Trump branded the "crimes of a monster."

As Trump embarked on a White House address to announce the action -- taken in defiance of Russia's threat to respond -- explosions were heard in the Syrian capital Damascus, signalling a new chapter in a brutal seven-year-old civil war.

AFP's correspondent in the city said several consecutive blasts were heard at 4:00 am (0100 GMT), followed by the sound of airplanes overhead. Smoke could be seen rising from the northern and eastern edges of the capital.

"A short time ago, I ordered the United States armed forces to launch precision strikes on targets associated with the chemical weapons capabilities of Syrian dictator Bashar al-Assad," Trump said, in a primetime address.

"A combined operation with the armed forces of France and the United Kingdom is now under way. We thank them both," Trump added.

"This massacre was a significant escalation in a pattern of chemical weapons use by that very terrible regime," he said of the suspected deadly gas attack a week ago on the rebel-held Damascus suburb of Douma.

"The evil and the despicable attack left mother 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 instead."

- 'Heavy strike' -

General Joseph Dunford, Washington's top general, said the precision strikes hit three targets -- a scientific research center near Damascus, a storage facility and command post also near the capital and a chemical weapons storage facility near Homs.

He added that Syrian surface to air missile batteries had attempted to fire back, but there were no initial reports of any allied losses.

It was a marked escalation in force compared to a US strike launched a year ago, when only cruise missiles were used against a single airfield.

Dunford said Russia's forces in Syria had been warned through existing "deconfliction" channels that western planes would be in Syrian air space, but that Washington had not revealed the target sites or timing in advance.

Defense Secretary Jim Mattis said no additional strikes were planned unless Assad again uses chemical weapons.

"We were very precise and proportionate, but at the same time, it was a heavy strike," he said.

- United front -

Trump also warned Russia and Iran not to stand by their ally in Damascus.

"Russia must decide if it will continue down this dark path or if it will join with civilized nations as a force for stability and peace," he argued.

The strikes had been expected since harrowing footage surfaced of the aftermath of the attack in Douma, which prompted a furious reaction from Trump.

Trump's anger and apparent determination was quickly matched by France's President Emmanuel Macron, who signed his country up for a joint response.

"We cannot tolerate the normalization of the use of chemical weapons," Macron said in a statement.

Britain's Prime Minister Theresa May was more cautious, but by the time the first precision cruise missile was launched, Trump had a mini-coalition.

"We cannot allow the use of chemical weapons to become normalized – within Syria, on the streets of the UK, or anywhere else in our world," May said, referring to a recent assassination attempt on a Russian double agent.

- 'Proof' -

In the days between the attack in Douma and the US-led response, Washington and Moscow clashed repeatedly in duelling press statements and US debates.

Moscow denied its ally Assad had any role in the outrage, pushing a variety of alternative theories that peaked with a claim that Britain had staged the event.

More practically, at the United Nations, Russia's diplomats vetoed a US motion to re-establish an international investigation into chemical weapons use in Syria that could have established blame.

Washington, Paris and London have nevertheless insisted that their own secret intelligence points to Assad's guilt, and on Friday, a US spokeswoman said they had "proof."

The western leaders apparently found this convincing enough reason to launch a punitive strike, but other observers are concerned that the crisis could escalate.

The Russian military had vowed to respond to any attack, and Russian President Vladimir Putin's administration had repeatedly warned that Trump was taking America down a dangerous path.

After the strikes, Moscow's ambassador to the United States, Anatoly Antonov, said: "We warned that such actions would not be left without consequences."

United Nations Secretary-General Antonio Guterres on Friday had warned the rival camps against "full-blown military escalation" and stressed the need to "avoid the situation spiraling out of control."

- Decision to act -

Trump has long criticized his predecessor Barack Obama for failing to enforce a previous US "red line" in 2013 after earlier chemical attacks blamed on Assad's forces.

And he set his own precedent just over a year ago when he ordered a cruise missile strike on a Syrian air base after sarin was fired at civilians in the town of Khan Sheikhun.

Backed by his hawkish new national security adviser John Bolton, who took office on Monday, Trump has been meeting with advisors and generals all week to plan.

Mattis had reportedly been arguing for a cautious response that would minimize the risk of the US being dragged deeper into Syria's civil war.

But other advisors wanted to use the opportunity to convince Trump, who wants to pull US forces out of Syria once the Islamic State jihadist group is defeated, to take a tough stance.

These hardliners, along with influential US allies Israel and Saudi Arabia, want Washington to counter Iran's growing power in Syria -- even if it means risking a perilous stand-off with Russia.

© Agence France-Presse