French television network TV5Monde was forced to broadcast only pre-recorded programmes on Thursday after it was hacked by individuals claiming to belong to the Islamic State group, who also hijacked its websites and social networks.

The Paris-based company was able to partially resume television broadcasts by 1 am (23:00 GMT) after a three-hour blackout, the network's director general Yves Bigot told AFP, but added its systems had been "severely damaged" by an "unprecedented attack".

On Thursday the French government denounced the cyberattack as an “act of terrorism”.

Prime Minister Manuel Valls said the hack was an "unacceptable attack on the freedom of information and expression", voicing "total solidarity with the editorial staff."

Minister of Culture Fleur Pellerin tweeted: “I give all my support and solidarity to the team at TV5Monde, victims of an obvious terrorist act.”

'We are facing determined terrorists'

Pellerin announced that she would bring together “representatives from the broadcast media and perhaps the written press” on Friday to look at ways of preventing this kind of attack.

France’s interior minister Bernard Cazeneuve said an investigation had been launched into the hacking of the channel.

“We are facing determined terrorists and we are determined to fight them,” said Cazeneuve.

TV5Monde is broadcast in more than 200 countries worldwide and is much better known outside France than it is at home, which could explain why the hackers decided to target the channel rather than TF1 or BFM TV.

The hackers posted documents on TV5Monde's Facebook page purporting to be the identity cards and CVs of relatives of French soldiers involved in anti-Isis operations, along with threats against the troops.

“Soldiers of France, stay away from the Islamic State! You have the chance to save your families, take advantage of it," read one message on TV5Monde's Facebook page. "The CyberCaliphate continues its cyberjihad against the enemies of Islamic State," the message added.

TV5Monde regained control of its social networks by 2:00 am Thursday but television broadcasts were likely to take hours, if not days, to return to normal. The attack would have required weeks of preparation, Bigot added.

Bigot told AFP earlier in the evening: "We are no longer able to broadcast any of our channels. Our websites and social media sites are no longer under our control and are all displaying claims of responsibility by Islamic State."

Its website was still offline by 14:00 on Thursday, displaying an "under maintenance" message to visitors.

'This is certainly a step up'

Close to 1,500 French nationals have left France to join the militants' ranks in Iraq and Syria, where they represent almost half the number of European fighters present, according to a report released Wednesday by the French Senate.

Jihadists have become increasingly adept at using the Internet to spread propaganda and attack media outlets.

In February, the Twitter feed of Newsweek was briefly hacked and threats were made against US President Barack Obama's family.

And in the immediate aftermath of the Charlie Hebdo attacks, hackers claiming to be Islamists hijacked hundreds of French websites, flooding them with jihadist propaganda.

"This is certainly a step up," said Gibert Ramsay, an expert on cyber-jihadism at St. Andrews University in Scotland.

"For years now, low-level cyber attacks have been a routine part of Islamist mobilisation. They have published manuals on how to hack websites. 

But this is an escalation," added the expert.