EU nations pledged Friday to step up co-operation to keep the more than 600 citizens now fighting in Syria from veering toward terrorism upon their return home.

EU home affairs ministers said they plan to increase surveillance of social media, intensify co-operation with nations that border Syria like Turkey and urge the European Parliament to approve legislation to make suspicious travel to the region more traceable.

French Interior Minister Manuel Valls says the number of EU citizens fighting abroad with potential links to terror groups “has never been seen at this scale.” He said it by far outstripped recent wars and conflicts in Afghanistan, Somalia and the Sahel region.

Valls said EU survey showed that some 500 citizens had left for Syria, and said the total for the EU’s 27 member nations would be higher than 600.

He refused to divulge specific numbers for other nations but said some 120 Frenchmen had already gone to Syria.

“In less than two years, the mass in Syria is bigger than that what we have known” in other recent conflicts combined, he said.

There are fears many EU citizens have joined extremist groups, sometimes with links to al-Qaida.

Once fully trained, radicalized or traumatized in the Syria conflict, these fighters could form an acute danger for their home nations upon their return.

Valls’ fears are shared by many countries, such as Britain, Germany, Spain, the Netherlands, Belgium and the Nordic nations.