Russia issued a strongly worded statement expressing its "deep concern" at a warning that Syrian rebels plan to target civilian airports in Damascus and Aleppo from Tuesday.
"In Moscow we have seen with deep concern the statements distributed in the media by representatives of the so-called Free Syria Army that international civilian airports of Damascus and Aleppo are from now on seen as military targets," the Russian foreign ministry on Monday said in a statement.
State-owned RIA Novosti news agency on Friday reported on a rebel statement cited by London-based Asharq al-Awsat newspaper saying Damascus and Aleppo airports and commercial flights would be targeted from Tuesday because they were being used for military aviation.
"We consider such threats absolutely unacceptable," the Russian foreign ministry said.
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"From a moral and legal point of view this means the opposition's critical proximity to a red line, beyond which are acts that are no different from the crimes of Al-Qaeda."
"The most recent statement by the Free Syria Army essentially confirms that terrorism is turning into one of its main methods of activity," the ministry added.
Russia called for "taking action on the leaders of the Free Syria Army in the most decisive manner to excuse such threats, not to mention their being carried out", and reiterated calls for Russians to avoid travel to Syria.
Rebel forces have increasingly targeted the Syrian regime's military air power and claimed to have shot down a MiG fighter jet and destroyed a dozen aircraft on the ground last week.
Russia continues to lobby for a short-lived agreement struck by world powers in Geneva on June 30 calling for a rapid ceasefire and supports a move towards a transition government that would decide President Bashar al-Assad's future.