The ongoing war in Syria, a magnet for Islamist fighters looking to participate in jihad, has attracted as many as 2,100 Westerners, a report revealed this week.

The Meir Amit Intelligence and Terrorism Information Center, a civilian organization run by former Israeli intelligence officers, noted a sharp rise in the number of Western recruits in the second half of 2013 and said that the fighters, mostly young men from Western Europe, increased the likelihood of religious war being brought home with fighters returning from the front.

“The foreign fighters in the ranks of the Al-Nusra Front and Islamic State [of Iraq and Greater Syria] are a potential threat to international security,” the authors wrote. Some will return home and “continue their terrorist and subversive activities” on their own initiative; some “may be handled by al-Qaeda…exploiting the personal relationships formed in Syria”; and some, like the veterans of Afghanistan, may sow terror internationally, traveling with a Western passport that raises fewer red flags.

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The authors put the total number of foreign fighters at between 6,000 and 7,000, with roughly 4,500 hailing from the predominantly Sunni states of Jordan, Saudi Arabia, Egypt and Libya. They estimated that 15-20 Israeli Arabs were engaged in the war and that, while the number of Palestinians was low, the trickle from Gaza has “risen sharply” over the past several months.

The Israeli fighters, the authors wrote, “veterans of the war in Syria, may be handled for espionage, subversion and terrorism” should they successfully mask their activities and manage to return to Israel. Palestinians from the Gaza Strip “may endanger both the de-facto Hamas administration and Egypt” and, surely, if their primary goal of wresting control of Syria is won, they “may increase the operational capabilities of Al-Qaeda-affiliated terrorist networks along Israel’s borders.”

The bulk of the Western nationals fighting in Syria are from Europe. The authors ranked Great Britain as the foremost source of recruits, with between 200 and 350 British nationals fighting in Syria. Belgium, Holland and Germany, too, were estimated to have produced over 200 recruits each, predominantly “young men who are second and sometimes third generation Muslim immigrants,” particularly from Pakistan and Morocco, according to the authors of the report.

Last week, the US director of National Intelligence, James Clapper, told the Senate Intelligence Committee that al-Qaeda-like groups operating in Syria are training fighters “to go back to their countries,” according to a recent AP report, and that they “have aspirations for attacks on the homeland.”

Senator Dianne Feinstein added that Syria could become “a launching point or way station for terrorists seeking to attack the United States.”

The Israeli intelligence center, using open sources, put the number of US nationals at “over 70” and wrote that the sharp rise, up from 12-15 earlier in 2013, “is indicative, in our opinion, of the significant rise in volunteers in the second half of 2013 (and perhaps to the increased attention paid to the phenomenon by the US security and intelligence officials).”

Calling the issue of foreign fighters “a global problem shared by the West, Israel and the Arab-Muslim world,” the authors wrote that international agencies have so far not developed effective methods to monitor, prevent and punish the volunteers.

“The returning foreign fighters are a ticking time bomb,” they wrote, “which can only be defused by international cooperation and joint systems to neutralize their terrorist-subversive potential.”