Red lines

Around the same time as he was musing about power to his friend Philippe Besson after his election victory last May, Macron also laid out a clear policy on using military force in Syria.

In the same month, with Russian President Vladimir Putin at his side, Macron said that further use of chemical weapons in Syria would be a "red line" that would trigger a response.

Having said Thursday that he had "proof" that Assad was responsible -- which is denied by the regime -- analysts saw Macron as having no choice but to go ahead with strikes.

"When you fix red lines, if you don't know how to make sure they are respected, you're choosing to be weak," Macron told The Guardian newspaper last June. "That's not my choice."

Many French officials still bristle as they recall how former US president Barack Obama had set a red line too over the use of chemical weapons in Syria, only to pull back at the last minute in 2013.

But in carrying through with his threat, Macron will be testing how far he can go against an intransigent Russia, which has denounced the strikes against Syria.

Macron, who has made no secret of his ambitions for playing a key role in Middle East diplomacy, is heading to Washington for a state visit this month, before a visit with Putin in Russia in May.

French Foreign Minister Jean-Yves Le Drian said Saturday that the Russia visit was still on despite the heightened tensions.

'Not taken lightly'

The Elysee Palace said the president wanted the UN Security Council to resume talks on Syria that Paris would "work seriously" with Russia.

"The UN Security Council must now resume, in unity, the initiative on the political, chemical and humanitarian aspects in Syria, to ensure the protection of civilians and for the country to finally find peace," it said in a statement on Saturday.

May spoke on the telephone to Macron and US President Donald Trump, her Downing Street office said, in which they agreed the strikes "had been a success".

A spokeswoman said: "The prime minister welcomed the public support which had been given by fellow world leaders for the strong stand the UK, France and the United States had taken in degrading Syria's chemical weapons capability and deterring their use; defending global rules; and sending a clear message that the use of chemical weapons can never become normalised."

For May, whose time in office has been consumed by extracting her country from the European Union, the Syria crisis presents a new headache.

British involvement in military interventions abroad is controversial in a country still haunted by its role in the 2003 US-led invasion of Iraq.

"There is no graver decision that a prime minister can take than to send service personnel into combat. It is a decision that I have not taken lightly," May told reporters on Saturday.

British lawmakers voted down taking military action against Damascus in 2013, in what was widely viewed as an assertion of parliamentary sovereignty on the use of force.

But in an earlier statement Saturday, May said there was "no practicable alternative to the use of force", insisting it had been "right and legal" to go ahead, and without consulting MPs.

A YouGov poll in The Times newspaper conducted this week found that 43 percent of voters oppose strikes in Syria, with 34 percent unsure and only 22 percent supportive. (AFP)