British Prime Minister David Cameron and French President held talks on defense and security collaboration that would see the two governments working together more closely on monitoring citizens and non-European residents who are already known to be in Syria or planning to join radical groups there.
Cameron and Hollande met at Royal Air Force Brize Norton in Oxford, about 65 miles northwest of London.
Neither side would say whether their joint work on security and counterterrorism measures would include other EU members, but Anglo-French collaboration builds upon pan-European links already in overdrive over the emerging threat of a jihadist spillover in Europe.
Britain and France are seen to be in the front line because of their active role in the toppling of Moammar Gadhafi's Libyan regime in 2011, proactive diplomacy aimed at the removal of Syrian President Bashar Assad, a tough stand on Iran's nuclear program and covert collaboration on containing a jihadist spread in Africa.
Hundreds of suspected jihadists are under surveillance in both countries, a few dozens are in preventive detention and more arrests are likely, officials say.
Officials cited between 600-700 Britons or British residents known to be active in Syria, a figure also mentioned by Hollande. French arrests were not discussed in news media reports. An earlier report by Germany's Der Spiegel cited about German jihadists known to be active in Syria.
A growing number of German jihadists are heading to Syria to join the rebels in their fight against Assad, Der Spiegel said. German intelligence sources cited by the news magazine said about 200 Islamists from across Germany had gathered in Syria last year.
The Le Monde newspaper in Paris, in an article that featured a map of movements radiating from Middle East conflict zones, cited "the multiplying paths of Jihad."
Western interventions in Afghanistan, Iraq, Libya and Mali after the September 2011 attacks on the United States had spread "anti-American hatred among hundreds of fighters," Le Monde said. Security agencies cited in the media now say that sentiment is increasingly targeting European participants in the conflict, in particular Britain and France.
British Foreign Secretary William Hague has warned the prolonged Syrian conflict has increased risk it will breed a new generation of battle-hardened militants posing a threat to Britain and other countries in Europe.
Belgian security agency reports said as many as 5,000 jihadists with European passports could be fighting alongside radical groups in Syria.
"We have to anticipate the returns (of fighters from Syria), the ways to handle this, the prevention measures and especially the exchange of information on the travels," Belgian Interior Minister Joelle Milquet warned in December.
Hague said the threat wasn't limited to EU member countries and non-EU countries would also be at risk, a reference to Russia. Russian officials and media say Russian Chechen rebels are among foreigners in al-Qaida rank and file fighting Assad. European security chiefs have told news media the situation is increasingly more complex and the foreign jihadists may no longer be adherents solely of al-Qaida or only anti-Assad groups.
Syria remains "by far the most attractive location for jihadists," a classified German intelligence report cited by Der Spiegel said. Of greater concern for European security experts, however, is what the jihadists will do when the Syrian conflict ends and the fighters return home to their respective countries.
Meanwhile, the trial of three French men accused of attempting to travel to Syria to join jihadists fighting in the country began in Paris Thursday, France 24 reported.
Youssef Ettaoujar, 26, Salah-Eddine Gourmat, 24, and 21-year-old Fares Farsi are accused of planning to go to Syria to procure weapons, undergo military training and go into combat.
French police placed the men under surveillance in 2012 and arrested them in May that year at an airport near Saint-Etienne, east-central France, as they attempted to board a flight to Turkey.