Israel warns Russia: Israel has been lobbying Moscow to drop the sale, fearing the missiles would upset the balance of power in the region.
Israel's defense minister on Tuesday signaled that his military is prepared to strike shipments of advanced Russian weapons to Syria, in a rare implied threat to Moscow.
Defense Minister Moshe Yaalon issued his warning shortly after a top Russian official said his government reserves the right to provide Syria with state-of-the-art S-300 air defense missiles.
Israel has been lobbying Moscow to drop the sale, fearing the missiles would upset the balance of power in the region and could slip into the hands of hostile groups, including the Lebanese militant Hezbollah, a close ally of the Syrian regime.
"As far as we are concerned, that is a threat," Yaalon told reporters when asked about the S300s.
"At this stage I can't say there is an escalation. The shipments have not been sent on their way yet. And I hope that they will not be sent," he said. But "if God forbid they do reach Syria, we will know what to do."
Since Syria's civil war erupted in March 2011, Israel has repeatedly voiced concerns that Syria's sophisticated arsenal, including chemical weapons, could either be transferred to Hezbollah, a bitter enemy of Israel, or fall into the hands of rebels battling Syrian President Bashar Assad. The rebels include al-Qaida-affiliated groups that Israel believes could turn their attention toward Israel if they topple Assad.
Israel has carried out several airstrikes in Syria that are believed to have destroyed weapons shipments bound for Hezbollah. Israel has not confirmed carrying out the attacks. The delivery of the Russian missiles to Syria could limit the Israeli air force's ability to act.
In Moscow, Russia's deputy foreign minister, Sergei Ryabkov, wouldn't say whether Russia has shipped any of the long-range S-300 air defense missile systems, but insisted that Moscow isn't going to abandon the deal despite strong Western and Israeli criticism.
"We understand the concerns and signals sent to us from different capitals. We realize that many of our partners are concerned about the issue," Ryabkov said. "We have no reason to revise our stance."
He said the missiles could be a deterrent against foreign intervention in Syria and would not be used against Syrian rebels, who do not have an air force.
"We believe that such steps to a large extent help restrain some 'hotheads' considering a scenario to give an international dimension to this conflict," he said.
Russia has been the key ally of the Syrian regime, protecting it from United Nations sanctions and providing it with weapons despite the civil war there that has claimed over 70,000 lives.
It is unclear if Moscow has already provided Damascus with any of the S-300s missiles, which have a range of up to 200 kilometers (125 miles) and the capability to track and strike multiple targets simultaneously.
Ryabkov's statement came a day after European Union's decision to lift an arms embargo against Syrian rebels. He criticized the EU decision, saying it would help fuel the conflict.
Israel's defense chief spoke at an annual civil defense drill to prepare for missile attacks on Israel. This year's exercise comes at a time of heightened concerns that Israel could be dragged into the Syrian civil war.
A number of mortar shells from the fighting in Syria have landed in the Israeli-controlled Golan Heights. WhileIsrael believes most of the fire has been errant, it has accused Syria of firing intentionally at Israeli targets on several occasions, and last week the sides briefly exchanged fire.
Israel's civil defense chief, Home Front Minister Gilad Erdan, said this week's drill was not specifically connected to the tensions with Syria.
"But of course we must take into consideration that something like that might happen in the near future because of what we see in Syria, and because we know that chemical weapons exist in Syria and might fall to the hands of radical Muslim terror groups," he said.