European Union home affairs ministers on Friday discussed the potential security threat posed by the hundreds of young Europeans believed fighting alongside rebels in Syria.

"They're not all jihadists or terrorists," said French Interior Minister Manuel Valls. "But we need to be very concrete in how we work together and we must do this operationally."

The EU's counter-terrorist coordinator Gilles de Kerchove said this week that while dozens of would-be jihadist fighters had left for theaters such as Afghanistan or Pakistan in the past, Europe had never faced a problem on a similar scale.

It was easy and cheap to reach Syria from Europe, with a bus ride from the Balkans for instance costing only around 100 euros.

Some 600 foreign fighters are believed to have left Europe for Syria to fight against President Bashar al-Assad's regime and "not all of them are driven by ideology," de Kerchove said.

EU nations needed to work on preventing people from leaving for the battlefront and preparing for their return should they be a security threat.

Spanish Interior Minister Alberto Ruiz Gallardon said his country was mulling legislation enabling the intrusion of terrorist suspects' computers.

Belgian counterpart Joelle Milquet said the EU needed to work with the United States and with Turkey, the main transit nation to Syria.

"Though security is a domestic issue, Europe can bring a plus by producing judicial norms," Valls added.

De Kerchove said EU countries could both share information and experience on how to track the process of radicalisation at grassroots level as well as monitoring social media to spot recruitment.

It was also key to work with Turkey, which was more than willing to cooperate, he said.

A recent report by King's College London has said up to 600 people from 14 European countries, including Austria, Britain, Germany, Spain and Sweden had taken part in the Syria conflict since it began in March 2011.