In its first major military response to Friday's attacks in Paris, France said 12 of its warplanes had hit IS positions in the city of Raqqa, the jihadists' de facto Syrian capital.
Activists and a monitoring group said the wave of strikes had shaken the city and sparked panic but the number of casualties was not immediately clear.
"There were at least 36 explosions overnight in Raqqa city, some caused by air strikes and some by weapons and explosives detonating after being hit," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
"The blasts shook the entire city," he told AFP.
France's defence ministry said the warplanes, including Rafale and Mirage fighters, had dropped 20 bombs on targets including a command post, a recruitment centre and arms depots south of Raqqa.
A training camp west of the city was also hit, the ministry said.
The Observatory could not immediately confirm any casualties in the strikes, which came after IS claimed responsibility for the attacks that killed at least 129 people in Paris.
"IS has imposed a security alert on the city, and it is difficult to confirm information about casualties from hospitals there," Abdel Rahman said.
He said IS had already imposed security measures in the city in the wake of Russian and US-led coalition strikes, including evacuating some headquarters and moving the families of foreign members to elsewhere.
FRENCH STRIKES COULD HELP: EXPERTS
An activist group, Raqqa is Being Slaughtered Silently, said the heavy raids caused "panic" among civilians and a power outage.
It could not give a toll in the strikes, but said no civilians appeared to have been killed.
Raqqa city is a regular target of strikes by US-led coalition aircraft, Syrian warplanes, and more recently Russian strikes, which were launched on September 30.
Signup to our newsletter and follow us on Facebook and Twitter!
Experts said France's strikes could be useful if they were based on solid information, but warned that intelligence gaps and the risks of civilian deaths have long been obstacles to targeting IS.
"If the French do have good intelligence on where they're targeting and they are doing it for good reason rather than to just lash out, then it could in the long term build into something useful," said IS expert and researcher Charlie Winter.
"But there is a big possibility that this is just air strikes driven by vengeance, which, while completely understandable, may not be the most pragmatic option."
He said IS's top leadership was unlikely to be in Raqqa, and that the group would seek to capitalise on the strikes by using any civilian casualties for propaganda.
"There are sure to be civilian casualties at some point if there haven't been already, and those civilian casualties are sure to be paraded in front of the camera," he told AFP.
GOING AFTER IS REVENUES
The US-led coalition has been carrying out strikes against IS in Syria since September 2014, expanding a campaign that began in Iraq.
The coalition has killed hundreds of members of the group, according to the Observatory, but has had a limited effect on its ability to hold territory.
On Monday, the Pentagon said coalition strikes on Sunday destroyed 116 fuel trucks used by the jihadist group near Albu Kamal, an IS-held town in Deir Ezzor province on the border with Iraq.
A spokesman for the coalition confirmed the strike was "the first time that we've hit so many (fuel trucks) at once".
He said the trucks had been parked, getting ready to be filled or depart to sell their contents.
IS reportedly makes millions of dollars in revenue from oil fields under its control and the US-led coalition has regularly targeted oil infrastructure held by the group.
The coalition has vowed to go after IS's financial resources, the spokesman said, adding: "This strike was part of that strategy to start degrading their financial ability."
Canada said its forces had also launched raids on IS targets in Iraq on Sunday -- as new Prime Minister Justin Trudeau comes under pressure in the wake of the Paris attacks to reconsider a promise to have Canadian forces in the coalition stop carrying out attacks.