Story highlights

Hackers claiming links to the group block 11 television channels, and take over website and social media accounts.

A French global TV network is back on air after hackers claiming allegiance to the Islamic State of Iraq and the Levant simultaneously blacked out 11 channels and took over its website and social media accounts.

French prosecutors opened an investigation into the attack that began late on Wednesday and blocked TV5 Monde from functioning for part of the day on Thursday. Operations were fully re-established by Thursday evening.

Bernard Cazaneuve, France's interior minister, told a news conference that "numerous elements converge to suggest the cause of this attack is, indeed, a terrorist act".

France is "absolutely determined to catch those who want to strike at its heart," the minister said.

Laurent Fabius, the French foreign minister, said the investigation would "get ahead of what the terrorists may have in their sick mind".

In what appeared to be the most ambitious media attack so far by the group, the hackers briefly cut transmission of 11 channels belonging to TV5 Monde and took over its websites and social media accounts.

The channel's director, Yves Bigot, said the attack continued into Thursday, however the station was able to broadcast a 6pm live show.

"We are no longer dark," the station said.

"Watch your back"

More than a dozen technicians worked to return the station to life "without erasing the traces of the intrusion, which are precious for the investigation," the National Agency for Computer Systems Security said in a statement.

The message on the TV5 Monde website read in part "I am IS" with a banner by a group that called itself Cybercaliphate.

Hackers operating under the name Cybercaliphate have carried out a string of attacks — including several in the US — since late last year.

Even though the hackers express support for the ISIL and routinely use its imagery in their attacks, it is not known for certain whether they are genuine members, simple supporters or hackers with no link to the group.

Experts who have followed the group's online communications say its supporters have regularly expressed interest in launching cyberattacks at Western targets.

Hackers claiming to have links to ISIL have previously seized control of the Twitter accounts of other media, such as Newsweek, and in January they hacked into the Twitter page and YouTube site of the US military's Central Command. They left a message at the Pentagon: "Watch your back".

William Reymond, the editor of the French investigative website Breaking3zero, which tracked the January hacks, said the latest attack could be directly linked to two ISIL fighters — one in Algeria who built the software and another in Iraq who helped speed up the attack.

Within 30 minutes, said Reymond, the malware had burrowed in and exploited a weakness to enter the network's computer system and take over its central transmission server, preventing the signal from being beamed to a satellite. He said TV5 Monde would have a hard time regaining full control.

"They have to erase everything. There were at least three other encrypted viruses," he said.

Cyber security expert Haroon Meer told Al Jazeera that such attacks were becoming common.

Source: Al Jazeera And AP