PARIS - Hackers claiming to be supporters of Islamic State knocked out channels of French public television station TV5Monde and posted material on its social media feeds to protest against French military action in Iraq.

President Francois Hollande's government branded the cyber attacks, which started late on Wednesday, a "vile and cowardly" affront to freedom of expression and began an investigation, promising to track down those responsible.

Yves Bigot, head of the TV5Monde station, said the assault caused its 11 channels to go temporarily off air and also hit its websites. The network broadcasts news and entertainment programmes in French on 11 channels around the world, and is 49 percent owned by state-backed broadcaster France Televisions.

Interior Minister Bernard Cazeneuve said it was too early to say where the attack had come from, but that cybersecurity measures would be reinforced.

"There are a lot of factors suggesting that a terrorist attack was behind this," he told a news conference.

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

The Facebook page was back up on Thursday but the main website remained down.

The French Foreign Ministry confirmed the attack had been claimed by individuals purporting to back Islamic State. French foreign, interior and culture ministers visited the channel's offices in Paris and pledged to identify those responsible.

A judicial source said a preliminary investigation had begun. After meeting with the heads of major broadcasters to assess weak spots and how best to deal with them, Culture Minister Fleur Pellerin said she had told them to be on the look out for new attacks.

"We told (them) to be vigilant without being excessively anxious on everything that could constitute a risk factor or vulnerability in their network," she said.

An official at French IT security agency ANSSI told Reuters the attack was particularly sophisticated and it would take several months to rebuild the broadcaster's network.

France is part of the international coalition carrying out strikes against Islamic State insurgents in Iraq. Its aircraft carrier Charles de Gaulle joined operations there in February.