Reuters reports: Israel is working with allies abroad to track Westerners fighting in Syria, concerned that such militants could attack Israeli or Jewish targets once back home, a senior Israeli official said on Tuesday.

Of an estimated 10,000 foreign combatants among rebels battling Syrian President Bashar al-Assad, around 20 percent are from the West and that number is rising, the official said.

“Think of a scenario, even one of them returning and getting instructions from someone he worked with, someone he fought beside, someone like (the al Qaeda-linked) Nusra Front, to carry out an attack,” said the official, who is privy to intelligence assessments. “This has been keeping us very, very busy lately.”

The official, who declined to be named because of the sensitivity of the issue, said Israel was coordinating monitoring efforts with Western countries, whose legal options against fighters returning home were limited.