Israel has quietly tested ways of defeating an advanced air-defense system that Russia has deployed in the Middle East and that could limit Tel Aviv's ability to strike in Syria or Iran, military and diplomatic sources said.

The sources said a Russian S-300 anti-aircraft system, sold to Cyprus 18 years ago but now located on the Greek island of Crete, had been activated during joint drills between the Greek and Israeli air forces in April-May this year.

The activation allowed Israel's warplanes to test how the S-300's lock-on system works, gathering data on its powerful tracking radar and how it might be blinded or bluffed.

One defense source in the region said Greece had done so at the request of the United States, Israel's chief ally, on at least one occasion in the past year. It was unclear whether Israel had shared its findings with its allies.

"Part of the maneuvers involved pitting Israeli jets against Greek anti-aircraft systems," one source said. Two other sources said the Crete S-300 was among the systems turned on.

The sources spoke to Reuters on condition they not be identified by name or nationality. The Greek and Israeli militaries declined to confirm or deny any use of the S-300 system during drills held in the Eastern Mediterranean last April-May or similar exercises in 2012 and 2010.

A senior Greek Defence Ministry official, asked whether the system was operating during Greek-Israeli military exercises, said: "At this moment the S-300 is not in operation." He said Athens' general policy was not to permit any other country to test the system's abilities.

The S-300, first deployed at the height of the Cold War in 1979, can engage multiple aircraft and ballistic missiles up to 300 km (186 miles) away. Israel is concerned by Russia's plan to supply S-300s to Iran.

Israel says Egypt, with which it has a cold peace, has bought a variant of the system. The Israelis also worry about Moscow's announcement last month that it will deploy the S-300 or the kindred system S-400 from its own arsenal in Syria, in response to Turkey's shooting down of a Russian jet there.

Israel has bombed Syrian targets on occasion and is loath to run up against the Russians. Israeli Prime Minister Benjamin Netanyahu has met President Vladimir Putin at least twice in recent weeks to discuss coordination and try to avoid accidents.