JERUSALEM -- Israel has lobbied Russia to pull away from selling a strategic air defense system to Iran but has received only vague assurances, Israeli defense sources said Monday.

Last week, Israel agreed to supply surveillance drones worth $50 million to Russia. The Israeli Haaretz newspaper said this followed a pledge by Moscow not to sell Iran the S-300, which could protect Iranian nuclear facilities against air strikes.

An Israeli defense official said he had no knowledge of such an undertaking by Russia in its talks with Israel on the matter. Moscow has given mixed messages on the prospect of Iran buying S-300s.

A recently retired Israeli official informed about bilateral security talks with Russia said that while Israel has made clear its objections about Iran receiving S-300s, "the Russians don't make promises of this kind."

"The most we have at this point is a vague assurance that the deal is not going ahead," the former official said. "But that could change at any time, and one of the relevant factors is Israel's policies on Iran."

An industry source said Israel hoped to win scope to apply pressure over Iran with the drone deal. "The thinking was: let's go ahead and sell and then try to gain sympathy on the S-300 issue," the source said.