TEL AVIV - Iran is set to receive an advanced Russian-made anti-aircraft system by year-end that could help fend off any preemptive strikes against its nuclear facilities, senior Israeli defence sources said on Wednesday.

First delivery of the S-300 missile batteries was expected as soon as early September, one source said, though it could take six to 12 months for them to be deployed and operable -- a possible reprieve for Israeli and American military planners.

Iran, which already has TOR-M1 surface-to-air missiles from Russia, announced last December that an unspecified number of S-300s were on order. But Moscow denied there was any such deal.

Washington has led a diplomatic drive to deny Iran access to nuclear technologies with bomb-making potential, while hinting that force could be a last resort. Israel, whose warplanes have been training for long-range missions, has made similar threats.

But the allies appear to differ on when Iran, which denies seeking atomic arms, might get the S-300. The most sophisticated version of the system can track 100 targets at once and fire on planes 120 km (75 miles) away.

"Based on what I know, it's highly unlikely that those air defence missiles would be in Iranian hands any time soon," U.S. Secretary of Defence Robert Gates said in a July 9 briefing when asked about the S-300 -- also known in the West as the SA-20.

An Israeli defence official said Iran's contract with Russia required that the S-300s be delivered by the end of 2008. A second source said first units would arrive in early September.

The official agreed with the assessments of independent experts that the S-300 would compound the challenges that Iran -- whose nuclear sites are numerous, distant, and fortified -- would already pose for any future air strike campaign by Israel.

Time to Learn

Israel does not have strategic "stealth" bombers like the United States, though the Israeli air force is believed to have developed its own radar-evading and jamming technologies.

"There's no doubt that the S-300s would make an air attack more difficult," said the official, who declined to be named.

"But there's an answer for every counter-measure, and as far as we're concerned, the sooner the Iranians get the new system, the more time we will have to inspect the deployments and tactical doctrines. There's a learning curve."

Israel, which is assumed to have the Middle East's only nuclear arsenal, carried out a large-scale air force drill over the Mediterranean last month which was widely seen as a "dress rehearsal" for a possible raid on Iran. Some analysts also described it as a bid to pressure the West to step up sanctions.

The exercise involved overflying parts of Greece, which is among a handful of countries to have bought and deployed S-300s. But Greek media quoted Athens officials as saying that the system's radars were "turned off" during the Israeli presence.

According to the Israeli official, it would take a year for Iran to deploy the S-300s and man them with trained operators.

Robert Hewson, editor of Jane's Air-Launched Weapons, said: "The minimum work-up time to be comfortable with the system is six months, but more time is preferable."

Hewson said the Iranian S-300 deal was being conducted via Belarus to afford discretion for Russia, which is already under Western scrutiny for helping Iran build a major atomic reactor.

"Belarus is the proxy route whenever Russia wants to deny it is doing the sale. But nothing happens along that route without Moscow saying so," he said.

(Additional reporting by David Morgan in Washington and Daniel Flynn in Athens; Editing by Catherine Evans)