Geneva - The "main battle" for the Syrian city of Aleppo is about to start, a top UN official said on Thursday, highlighting the spiralling violence in the conflict.
UN peacekeeping chief Herve Ladsous also said after briefing the UN Security Council on the conflict that UN observers had not yet seen opposition rebels using the tanks and other heavy weapons that they now have.
"The spiral of violence is still increasing," Ladsous told reporters.
"The focus two weeks ago was on Damascus, the focus is now on Aleppo where there has been a considerable build-up of military means and where we have reason to believe that the main battle is about to start," he added.
Aleppo has been under siege from President Bashar Assad's forces since 20 July and military observers have predicted a prolonged battle for the northern city.
Syrian rebels used tanks for the first time to attack a military airport northwest of Aleppo on Thursday, a rebel commander said. Rebel commander Abdel Aziz Salameh said his forces had captured four tanks from government forces.
Number of observers cut
Ladsous said unarmed military observers who have been near Aleppo have seen the heavy weapons.
"We have not yet seen the opposition in the action of using those heavy weapons against government forces. But we know that they have tanks, that they have armoured personnel carriers et cetera - that's a fact," he said.
The UN Supervision Mission in Syria has cut its number of observers from nearly 300 to less than 150 in the past two weeks because of the increased strife and the suspension of most of its operations.
Ladsous said the mission had still been able to carry out about 50 patrols in the past two weeks and had tried to arrange local ceasefires to let civilians get out of conflict zones.
UN Secretary General Ban Ki-moon is preparing recommendations on the future of the UN presence in Syria after the UNSMIS mandate ends on 20 August.
Western nations want the mission closed down while Russia is arguing that it should be extended and strengthened, despite the increased violence and the resignation of UN-Arab League envoy Kofi Annan.
Russia's UN ambassador Vitaly Churkin said UNSMIS was still playing an "important and useful role".
France's UN ambassador Gerard Araud said the observers will probably have to leave because there is no agreement on the 15-nation Security Council about the future of UNSMIS.
"Honestly, I think there will be no agreement. I think the mission will disappear on August 19," Araud told reporters. "I cannot see a scenario, except a change on the ground, that would allow the mission to stay."