European police agency Europol said Monday it was launching a continent-wide cybercrime unit to combat social media accounts promoting jihadist propaganda, particularly those of the Islamic State (IS) group.

The unit, set to start operating from Europol's Hague-based headquarters next month, will comb tens of thousands of social media accounts connected with IS and report them to the companies behind the websites, Europol chief Rob Wainwright said.

He declined to name Facebook and Twitter "for privacy reasons," but said: "These are the leading social media companies. There's only three or four, so that's who we are talking about."

The team "will focus on publicly-available material and combine what we see on social media with more traditional intelligence sources," Wainwright told AFP in a telephone interview.

Initially consisting of some 15 to 20 members, the cyber squad will focus on key figures who put out thousands of tweets and run accounts used to lure would-be jihadists to Iraq and Syria, as well as to recruit jihadists' brides.

A recent US study identified at least 46,000 Twitter accounts linked to supporters of the IS group, three-quarters of them tweeting in Arabic.

Since the IS group called on Muslims to come to the caliphate it declared a year ago, foreign fighter numbers have jumped, with the United Nations reporting a 71 percent spike in the nine months to April.

The International Centre for the Study of Radicalisation in London said the number of foreigners fighting in Syria and Iraq topped 20,000 by January -- with nearly a fifth of them from western Europe.

"The IS is the most well-connected terrorist organisation that we've seen online," Wainwright pointed out.

"They are manipulating the Internet and social media, which has become a cornerstone in the lives of many young people," he said.

Europol will draw on a decade of experience in monitoring extremist websites and well as "deep knowledge of extremist content and good linguistic capabilities including our knowledge of Arabic," to combat the problem.

Wainwright said once an extremist account had been detected, the companies would be informed and it would be taken down in "a matter of a few hours."