France said Monday it had begun preparations for air strikes on the Islamic State (IS) in Syria, and Britain said it too was weighing the case for hitting at jihadists in the war-torn country, from which tens of thousands of people have fled to Europe.

British Prime Minister David Cameron emphasised the risk of terror attacks in saying he believed there was "a strong case" for Britain taking part in airstrikes in Syria.

Britain had carried out a first strike in Syria in August with a drone, killing three jihadists, two of them from Britain, he revealed.

The RAF drone strike that killed Reyaad Khan and another Briton was "entirely lawful" because Khan had been planning and directing "specific and barbaric" attacks in Britain, Cameron said.

"Is this the first time in modern times that a British asset has been used to conduct a strike in a country where we are not involved in a war? The answer to that is yes," he added.

Britain is already carrying out airstrikes on IS targets in Iraq, but Cameron said he wanted Britain to able to extend its bombing campaign to Syria.

In Paris, President Francois Hollande said he had ordered surveillance flights to begin over Syria to lay the ground for air strikes on IS targets.

"I have asked the defence ministry that from tomorrow surveillance flights can be launched over Syria, allowing us to plan airstrikes against Daesh (the Islamic State group)," Hollande told a press conference.

France has been targeted by a series of jihadist attacks this year, most recently when a heavily armed suspected extremist tried to attack a train on French soil only to be overpowered by passengers.

Coupled with the migrant crisis, which is partly fuelled by refugees fleeing the chaos in Syria, the political pressure has been building on Hollande to take action.

Like Britain, France is currently only participating in missions against IS in Iraq.

"What we want is to know what is being prepared against us and what is being done against the Syrian population," Hollande said, referring to the reconnaissance flights.

He ruled out however sending French ground forces into Syria, saying it would be "unrealistic."

"It's for regional forces to take their responsibilities. France, however, will work to find political solutions," he said.

He said finding a political "transition" that sidelined Syrian President Bashar al-Assad was "essential".

- 'Late in the game' -

Analysts were lukewarm about Hollande's strategy shift.

"It's above all else a domestic political gesture, with in the background the message 'look, we're doing something'," retired French general Jean-Claude Allard, director of research at the Institute of International and Strategic Relations (IRIS), said.

Myriam Benraad, of the Centre of International Research (CERI) in Paris, said: "It's a direct response to this disaster, but it comes a little late, and airstrikes are not enough to solve this problem."

Cameron told parliament he believed the case for striking at IS in Syria "grows stronger with the growing number of terrorist plots".

Six plots against Britain had been uncovered over the past 12 months, he said.

In 2013, Cameron suffered a humiliating defeat in a Commons vote on military action against Assad's regime, after a large-scale chemical weapons attack blamed on Syrian government forces.

Because of his slim parliamentary majority, and because some of his own lawmakers are against the move, Cameron would need opposition backing for expanding the campaign against IS.

Jeremy Corbyn, the founder of the Stop the War coalition who is the favourite to win the leadership of the main opposition Labour Party this week, is vehemently opposed to airstrikes.

"My view is that it would create more problems than it would solve," he repeated Monday.

The initiatives come amid growing concern in the West over reports that Russia is increasing its military support to Assad's regime.

Moscow has been a bulwark of support for the regime since Syria's civil war began in 2011.

A Greek foreign ministry official said Monday that the US had asked Greece to bar Russian supply flights to Syria from its airspace.

"We received the (US) request on Saturday and are examining it," the official said on condition of anonymity.

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Cameron International