Beirut (AFP) - The Islamic State group said Wednesday it had killed a Chinese and a Norwegian hostage, as French and Russian air strikes on its Syrian stronghold were reported to have left 33 fighters dead.

Moscow announced that its warplanes were hunting IS oil tanker trucks in Syria, a day after saying a "terrorist attack" brought down a Russian passenger jet over Egypt last month, killing all 224 people on board.

Those deaths and the shootings and suicide bombings in Paris were claimed by the Islamic State group, which declared a "caliphate" in Iraq and Syria last year.

On Wednesday, IS said it had killed the two hostages.

Its English-language Dabiq magazine featured graphic photos of two bodies that appeared to be Chinese hostage Fan Jinghui and Norwegian Ole-Johan Grimsgaard-Ofstad.

A stamp-like caption overlaid on the full-page photo read, "Executed after being abandoned by the kafir (disbeliever) nations and organisations."

It was unclear when, where, or how they were killed, but their heads were bloodied by apparent gunshot wounds.

The Norwegian prime minister's office said the photos "seem to show that the hostage Ole-Johan Grimsgaard-Ofstad was executed. We are still verifying it."

Meanwhile, China "noticed the report and was greatly shocked," said foreign ministry spokesperson Hong Lei in a statement, adding that the information needed to be further verified.

The two men were last featured in Dabiq's September edition, in which IS published an "advertisement" that they were "for sale".

The magazine also featured an article entitled "Paradigm Shift II" allegedly penned by British hostage John Cantlie as a sequel to the last piece published under his name in Dabiq's March issue.

The essay allegedly written by Cantlie came after a long absence of his "work", which regularly featured in the magazine.

Cantlie claims media outlets, security services and experts themselves are acknowledging that IS "is a genuine state".

- IS in 'civilian homes' -

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Since Sunday, Russian and French raids have struck arms depots, barracks and other areas in Raqa city, the jihadists' stronghold in northern Syria.

"This is where we must hit Daesh, in its lifeblood," said French Defence Minister Jean-Yves Le Drian, using the Arabic acronym for the group.

A preliminary death toll from the Syrian Observatory for Human Rights monitor said 72 hours of strikes "left 33 dead and dozens wounded in IS ranks".

Russia said its air force had destroyed some 500 fuel trucks in the past few days as they transported oil from Syria to refineries in Iraq in what is a key part of IS financing.

The Pentagon said Moscow warned Washington of its impending attacks on Raqa. This was to avoid any US planes in the area being endangered, spokesman Peter Cook said, which "wasn't necessary in this case".

Aktham Alwany, a journalist and activist from Raqa, said civilians in the city were "only moving around when necessary" out of fear of strikes by "whichever nationality -- Russian, regime, coalition".

"Unfortunately, it's no secret that IS's bases are inside civilian homes. There are some bases that look like they're for IS, but in reality they're empty fakes, while civilian homes are teeming with them," Alwany told AFP.

Raqa was Syria's first provincial capital lost by the government, seized by rebels in 2013 then overrun by IS in January 2014.

At least 300,000 people live there now, according to analyst Fabrice Balanche.

- Assad 'lesser evil' -

IS's speedy expansion sparked a US-led air coalition to begin targeting it in both Iraq and Syria, with French strikes on the latter beginning in September.

Moscow launched its own air war in Syria, in coordination with President Bashar al-Assad, on September 30.

Despite their diametrically opposed stances on Assad, France and Russia agreed to coordinate their military and security services to fight IS after the attacks in Paris and the downing of the Russian airliner.

French Foreign Minister Laurent Fabius said Wednesday aircraft carrier Charles de Gaulle will be operational in the eastern Mediterranean "by the end of the week".

And US President Barack Obama praised Russia as a "constructive partner" in international talks in Vienna aimed at reaching a solution to Syria's bloody conflict, which has cost 250,000 lives.

The US and France have been firm backers of Syria's uprising, while Russia and Iran remain staunch allies of Assad.

Russian Foreign Minister Sergei Lavrov said it would be "simply unacceptable" to set Assad's departure as a precondition to "fight against terror".

Although profound policy differences remain, IS's attacks have shifted international focus on to the jihadist group.

Turkish Foreign Minister Feridun Sinirlioglu said Ankara "has plans" for a joint operation with the United States to root out IS's presence along its border with Syria.

And Spain's foreign minister, Jose Manuel Garcia-Margallo, said engaging with Assad was a "lesser evil" and necessary to reach peace.