The main battle for the Syrian city of Aleppo is imminent, a senior UN official has said, hours after Kofi Annan quit trying to end the conflict.
Peacekeeping chief Herve Ladsous warned of intensified fighting in the civil war after rebels obtained tanks and other heavy weapons.
"The spiral of violence is still increasing," he said.
"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."
Aleppo has been under siege from President Bashar al Assad's forces since July 20, using jets and artillery to target positions within the northern city, and military observers have predicted a prolonged battle for the commercial hub.
The warning came as rebel forces, who hold several areas of the country's largest city, used tanks for the first time to attack a military airport to the northwest of the city, according to a commander.
Mr Ladsous said unarmed military observers near Aleppo had seen the heavy weapons in the hands of the opposition.
"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 etcetera. That's a fact."
Fighting has also continued to claim lives elsewhere in the country. In Hama, Syrian forces killed at least 50 people, including 21 members of three families on Thursday, according to local activists and residents.
"During the clashes the army entered the neighbourhood of Arbaeen and conducted raids, during which they killed members of three families," resident Abu Ammar told reporters.
South of the capital Damascus, a raid by Syrian forces killed another 43 people, according to the London-based Syrian Observatory for Human Rights.
"Regime forces entered the Jdaidet Artuz district on Wednesday and arrested around 100 young people who were taken to a school and tortured," the opposition group said in a statement.
"On Thursday morning after the operation the bodies of 43 people were recovered. Some of them had been summarily executed."
Meanwhile, China has expressed regret over Mr Annan's decision to stand down.
The UN-Arab League envoy to Syria tendered his resignation after complaining that his April peace plan had not received the support it needed from major powers.
He also hit out at "continuous finger-pointing and name-calling" at the UN Security Council which he said had prevented co-ordinated action to end the bloodshed.
His resignation sparked a new round of recriminations among the five permanent members of the UN Security Council, with Washington blaming Beijing and Moscow's vetoing of three separate resolutions on the Syrian conflict.
"China expresses regret at Annan's resignation. We understand the difficulty of Annan's mediation work, and respect his decision," foreign ministry spokesman Hong Lei said in a statement.