Iran’s ally Hezbollah is paying former U.S.-backed rebels to switch sides and join a growing force in southern Syria, deepening its presence near Israel’s border after appearing to withdraw to avoid Israeli airstrikes, according to activists and a former rebel commander.

The Iran-backed militia has recruited up to 2,000 fighters, these people said, most of them from rebel groups that lost U.S. funding last year, according to the former commander, who tracks recruitment in villages in southern Syria.

The Syrian government and its military ally Russia are depending on Hezbollah and other Iran-allied militias to fight the remaining armed opposition in the south, chiefly Islamic State.

Israel, which views Iran as an existential threat, has warned it won’t allow forces loyal to Iran to entrench near its border.

Lebanon-based Hezbollah’s recruitment of fighters in southern Syria is “a highly destabilizing prospect,” said U.S. Syria Envoy Joel Rayburn. “The idea that Hezbollah would be expanding its presence down there on the Jordanian frontier, near the Golan Heights, near the Israeli frontier. This would increase the chance for conflict,” Mr. Rayburn said Friday during a conference in Manama, Bahrain.