The US and Germany have called on Russia not to supply Syria's military with an advanced missile system they say could prolong the conflict there.
US Secretary of State John Kerry said the delivery of Russian weaponry would have a "profoundly negative impact" and put Israel's security at risk.
German Foreign Minister Guido Westerwelle urged Moscow not to hinder the chances of mooted peace talks.
In Syria, fierce fighting continued on Friday around the key town of Qusair.
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