UN-Arab League envoy Kofi Annan told the UN Security Council on Thursday that the Syria crisis will "spiral out of control" unless substantial pressure is put on President Bashar al-Assad, diplomats said.
The use of heavy weapons and surveillance drones against UN monitors has raised questions meanwhile about how long a UN mission can be kept in the country, UN leader Ban Ki-moon told the council.
Annan renewed calls for the major powers to warn Assad of "clear consequences" if he does not comply with a six-point international peace plan, one diplomat inside a closed-door council briefing told AFP.
"The longer we wait, the darker the future looks for Syria," the international envoy also told the 15-member council, another diplomat quoted Annan as saying.
The council must apply "united" and "substantial" pressure on Assad, the international envoy added. Annan again highlighted that his peace initiative could not be "open-ended".
The Security Council has passed two resolutions which approved the UN monitoring mission in Syria and condemned the violence there. But it is divided over how to increase pressure.
Russia, Syria's last major ally, and China have vetoed two council resolutions which only hinted at future sanctions. The United States and European nations want economic sanctions on Assad.
Heavy weapons, armor-piercing bullets and surveillance drones have been used against UN observers to hamper their efforts to monitor the worsening conflict, UN leader Ban told the meeting.
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The tactics had been used to try to force the unarmed monitors to withdraw from areas where government forces have been accused of staging attacks, Ban was quoted as saying.
Ban said the heavy shelling had been used to deter a UN Supervision Mission in Syria (UNSMIS) convoy, drones had monitored the movements of observers and the armor-piercing bullets had been fired at UNSMIS vehicles.
According to UN officials, UN vehicles are shot at almost every day in Syria. At least two roadside bombs have also targeted UN convoys since the mission started.
Ban praised the "brave" monitors but said the Security Council would have to consider whether the mission is "sustainable".
UN observers had seen Syrian military convoys approaching villages and tried to stop tank assaults against populated areas, but had been "ignored", Ban told the meeting which was held only hours after a new massacre in which dozens of people were reported killed.
Ban said shots were fired at the UN convoy which tried to get into the village of Al-Kubeir to investigate the slaughter.
Ban said that according to preliminary evidence, the Syrian army had surrounded the village and militia had entered Al-Kubeir and killed civilians with "barbarity", according to diplomats at the meeting.
UN monitors had been prevented from going into Al-Kubeir but would make a new attempt to enter the village on Friday, UN spokesmen said.