JERUSALEM — 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 Assad, around 20 percent are from the West and that number is rising, the official said.

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"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-Qaida-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.

"It's a problem [for Western authorities] to come and arrest someone just because he was in Syria. No one knows for sure what he did there," the official said.

Security sources said Israeli officials monitoring Syria were meeting foreign counterparts more often, and Israeli diplomatic missions were scrutinizing visitors more closely.

The Israeli estimates for the number of Western combatants in Syria largely correspond to those cited in a report last week by the International Center for the Study of Radicalization, a partnership of five universities based at King's College London.

The almost three-year-old Syrian civil war has indirectly helped Israel by diverting its Lebanese Hezbollah enemy, which has sent many guerrillas to fight for Assad's army. Hezbollah battled Israel to a standstill in the 2006 Lebanon border war.

The official said Israel believed between 10,000 and 15,000 Hezbollah members have fought in Syria, but that it was not clear how many casualties the Iranian-backed group had suffered.

"There were many killed, wounded, but there are no numbers," the official said. "It could be hundreds or thousands."

"In all, their capabilities against Israel have been hurt. So their interest, for now, in opening a front against Israel is near zero," the official said.

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