Israel has asked Russia not to sell Syria an advanced air defense system which would help President Bashar Assad fend off foreign military intervention as he battles a more than two-year-old rebellion, Israeli officials said on Thursday.

Israel told told Washington that Syria had already began payments for a 900 million Euro purchase of the S-300, and an initial delivery was due within three months.

U.S. Secretary of State John Kerry, meanwhile, said Thursday that the transfer of advanced missile defense systems from Russia to Syria would be a "destabilizing" factor for Israel's security.

Kerry says the U.S. has expressed concerns about what the S-300 batteries in Syria would mean for Israel's security. He wouldn't address what the missiles might mean for Syria's civil war.

The S-300 is designed to shoot down planes and missiles at 200 km ranges. It would enhance Syria's current Russian-supplied defenses, which did not deter Israel from launching air strikes around Damascus last weekend.

"We have raised objections to this (sale) with the Russians, and the Americans have too," an Israeli official told Reuters.

There was no immediate comment from Moscow or Damascus.

The government of Syrian President Bashar Assad has been seeking to purchase the advanced S-300 missile batteries, which can intercept both manned aircraft and guided missiles, from Moscow for many years.

The paper said the package included six launchers and 144 operational missiles, each with a range of 200 kilometers, with an initial shipment expected in the next three months.

While the effectiveness of Syria's aging air force is unclear, most experts believe that its air-defense missile system, which was upgraded after an alleged Israeli strike in 2007 on a suspected nuclear site, remains quite potent.

Western nations have repeatedly urged Russia to block the sale, which they argue could complicate any international intervention in Syria's escalating civil war.

In 2010, Russia backed out of a tentative S-300 sale to Iran that had been in the works for years. Russian President Dmitry Medvedev cited U.N. sanctions imposed that year over Iran's defiance of international demands to curb its nuclear program.

Israel and the United States, which threaten military attacks on Iranian nuclear facilities if diplomatic alternatives fail, had lobbied Moscow to drop the deal with Tehran.

Robert Hewson, an IHS Jane's air power analyst, said that were Syria to receive the S-300 it would probably take several months to deploy and operate the system. But he suggested it would not pose a big challenge for Israel's hi-tech air force.

"It's a fairly well-established, fairly well-understood system, so there is a corpus of knowledge, particularly among Israel's friends, about how to deal with this system," he said.

Once activated, the S-300 could easily be spotted thanks to its distinctive radar signal, Hewson said, "and from there it's a fairly short step to taking it out. It's not a wonder-weapon."

Cyprus bought the S-300 and eventually positioned it on the Greek island of Crete. Israel, which has close ties with Nicosia and Athens, may have tested its jets against that S-300's capabilities during Mediterranean overflights, Hewson said.

Open gallery view An archive photo of an S-300 air-defense missiles launcher, left, and a S-300 missiles guidance station, right, at an undisclosed location in Russia. Credit: AP