Facing military setbacks in its self-declared caliphate in Syria and Iraq and intensified air strikes from a US-led coalition, Islamic State may have decided in September to take the fight to France and elsewhere.

The ultra-hardline group has frequently threatened to strike inside Western countries since it established itself amid Syria's civil war and then spread to northern Iraq last year, but one fighter reached inside Syria said its spokesman Abu Mohammad al-Adnani had issued an instruction to act abroad.

"He sent a written order to all sectors and security brigades to start moving, including in Lebanon and Turkey," the Syrian IS fighter said via social media from northern Syria.

"Lebanon and France and other places are all part of the operations ordered two months ago."

Islamic State has said it was behind Friday's killings of at least 132 people in Paris in revenge for France's air strikes against it as well as twin suicide bombings which killed 43 people on Thursday in a Beirut stronghold of Lebanon's Shi'ite Hezbollah, which is fighting the group in Syria.

The ultra-hardline militants have also claimed responsibility for bringing down a Russian airliner over the Sinai Peninsula on Oct 31 which killed all 224 people on board after Russia began its own campaign of air strikes in Syria.

Turkish authorities suspect a high-profile British jihadist detained in Turkey last week may have been planning attacks in Istanbul similar to those in Paris, two security sources told Reuters on Sunday.

The group has also threatened to attack Saudi Arabia, United States and Russia.

It was not immediately possible to verify the reported order, which Islamic State supporters and fighters said was given to dormant cells in several places.

"Their messages to us are sent by blood and carnage so we send them their messages back in the same way, it is simple," the northern-Syria-based fighter said.

The group operates in a very secretive way and has a complicated structure. In general, its Caliph Abu Bakr al-Baghdadi is the ultimate decision maker but his deputy is also powerful. They both consult a Shura Council which is compromised of military, religious and other leaders who give advice to Baghdadi on strategy and military plans.

It practices a strict version of Islam which considers all those opposing it as infidels who should be killed.

The younger jihadi generation looks up to Baghdadi as a powerful leader who will help establish a greater Islamic State that will conquer the world to spread Islam.

It has drawn thousands of jihadists from across the world including Europe. But tighter security restrictions imposed by several European countries have prevented would-be jihadists from travelling and joining the group in Syria and Iraq.

To overcome this, the group has established contacts from its bases in the Middle East with these jihadists and encouraged them to operate as "lone wolves" or in small cells to carry out individual attacks inside countries where they live or work.

According to one of the fighters, the dormant cells have no contact with each other but all answer to a special apparatus in charge of "foreign operations", from which they take orders to attack. He did not elaborate.

Little is known about the head of this apparatus, who the fighter said is a Jordanian national who works closely with the leadership in Syria and Iraq and travels between the two countries. He is only known by a nickname.

"He masterminds these operations, gets in touch with the followers and supporters there, guide them in training and operations and targets," said a jihadi source close to the group.

His account could not be independently verified. The New York Times cited officials on both sides of the Atlantic saying that the attackers in France had communicated at some point beforehand with known members of Islamic State in Syria.

IS fighters said the Paris attacks had raised morale within Islamic State after a week in which it lost a strategic town in Syria close to the Iraqi border as well as the Iraqi town of Sinjar in one of the biggest counter attacks since IS swept through northern Iraq last year.

The Syrian army and its allies, including Hezbollah fighters backed by Russian air strikes, also broke a nearly two-year-old Islamic State siege of a Syrian army military base and freed soldiers who had been trapped there.

While Islamic State has frequently threatened to strike inside Western countries, its supporters say their battle with France, in particular, is a priority where they say Muslims were discriminated against.

"This is just the beginning. We also haven't forgotten what happened in Mali," said a non-Syrian Islamist fighter in Syria reached online, referring to the French-led military intervention in the West African country in 2013 against Islamist insurgents Paris said could launch attacks in Europe.

"The bitterness from Mali, the arrogance of the French will not be forgotten at all," he said, welcoming the Paris attack.

"Their atrocities in Syria and their support for tyranny is adding to it."

France said three jihadist cells staged co-ordinated hits on Friday night at bars, a concert hall and football stadium.

Prosecutors have said the slaughter appeared to involve a multinational team with links to the Middle East, Belgium and possibly Germany as well as home-grown French roots.

Islamic State, which captured swathes of territory in Iraq and Syria in 2011, has been suffering from an increased campaign carried out by U.S.-led coalition and Russia separately against its bases, training camps and officials.

Turkey has also come under international pressure to tighten its border to check the flow of foreign fighters coming to join Islamic State ranks.

In one of its popular anthems, called "Soon, soon", the group promises a long battle that will strike the heart of all those opposing it.

"Soon, soon you will be seeing wonders with a terrifying struggle. You will see. Our battles will be inside your home. I have drawn my sword for your destruction."

"You have started fighting me with this deluded alliance so now you will have a taste of my wrath. It will last for a long time. We will be coming to you with death and slaughter. And you will taste defeat."

Anti-Western sentiments grew dramatically among the group's supporters after U.S.-led coalition began its strikes against it in Syria and Iraq. They say they will not budge.

"We work based on an ideology. How can you defeat an ideology or someone who is a believer? The bigger their war against us, against Islam, the true Islam, the stronger our faith and commitment to our Caliphate grows," one supporter said.

"The State does not say what it is going to do tomorrow. It told the world that it will be punished and so it will be."