World powers traded blame on Friday after Kofi Annan quit as international peace envoy to Syria, complaining that his initiative to end the bloodshed there never received the support it deserved.
As Syria's government deployed fighter jets against rebels armed with tanks around the commercial capital Aleppo, the outgoing UN-Arab League envoy voiced regret at the "increasing militarisation" of the nearly 17-month conflict.
The former UN secretary general hit out at "continuous finger-pointing and name-calling" at the UN Security Council, which he said had prevented coordinated action to stop the violence.
"I did not receive all the support that the cause deserved," Annan told a hastily arranged news conference in Geneva.
"You have to understand: as an envoy, I can't want peace more than the protagonists, more than the Security Council or the international community for that matter.
"The increasing militarisation on the ground and the lack of unanimity in the Security Council fundamentally changed my role."
But Annan predicted that President Bashar al-Assad would go "sooner or later," and did not rule out his successor having more luck or success, despite his warning there was "no Plan B."
Writing in the Financial Times, Annan called on Moscow and Washington to shoulder responsibility for saving Syria from catastrophic civil war.
He stressed that Western military intervention would not deliver success on its own and that a political solution which was not comprehensive was doomed to fail.
"Syria can still be saved from the worst calamity. But this requires courage and leadership, most of all from the permanent members of the Security Council, including from Presidents (Vladimir) Putin and (Barack) Obama," he wrote.
Annan's resignation sparked a new round of recriminations among the council's five permanent members, with the United States blaming Russia and China for vetoing three separate UN resolutions on the conflict.
"Annan's resignation highlights the failure at the United Nations Security Council of Russia and China to support meaningful resolutions against Assad that would have held Assad accountable," said White House spokesman Jay Carney.
Germany also said Annan's decision was partly due to Chinese and Russian opposition to sanctions.
But Russia's envoy to the world body, Vitaly Churkin, insisted Moscow had supported Annan "very strongly," and Putin called his resignation a "great shame."
"Kofi Annan is a man of great merit, a brilliant diplomat and a very honest person, so it is a great shame," Putin was quoted as saying in London.
China said it wanted the United Nations to play an important role in trying to stop the conflict, which activists say has cost the lives of more than 20,000 people since it erupted in March 2011.
"China expresses regret at Annan's resignation. We understand the difficulty of Annan's mediation work, and respect his decision," said foreign ministry spokesman Hong Lei.
"China supports the UN playing an important role in the appropriate resolution of the Syrian issue."
Analysts say Beijing's unwillingness to back further action in Syria may stem from its discomfort with Western military intervention after last year's uprising in Libya, which eventually led to the fall of Moamer Kadhafi.
Russia and China are also expected to vote on Friday against a resolution before the UN General Assembly, where no country has the power of veto.
The resolution drawn up by Arab states deplores both the Syrian government's growing use of heavy weapons and world powers for failing to agree on measures to end the conflict.
Arab nations have dropped an explicit demand for Assad to quit, however, toning it down in an attempt to secure as large a majority as possible in the 193-member assembly.
On the ground, violence persisted across Syria and the battle for control of Aleppo intensified, with Assad's forces hammering rebel-held areas with fighter jets.
Rebels hit back by shelling the Menagh air base outside the northern city, and used tanks for the first time in the assault, a commander said.
"We had already attacked the airport several times, but this was the first time we used heavy weapons," which were "four tanks taken from Anadan," Abdel Aziz Salameh told AFP.
An AFP reporter who saw the bombardment said the rebels told him it was "an attack to take this airport being used by helicopters and planes that are firing on Aleppo."
Elsewhere, shelling of the Yarmuk Palestinian refugee camp on the southern outskirts of Damascus killed at least 21 civilians, including two children, the Syrian Observatory for Human Rights said on Friday.
Observatory director Rami Abdel Rahman said the shelling occurred as clashes flared between government troops and opposition fighters in the nearby Damascus neighbourhood of Tadamun.
"We demand an international investigation. We do not know the origin of the shelling," Abdel Rahman told AFP in Beirut by telephone.
The Observatory reported at least 179 people killed in violence on Thursday -- 110 civilians including 14 children, 43 soldiers and 26 rebels.
It also reported heavy shelling overnight around Houla, a town in the central province of Homs where at least 108 people were massacred at the end of May, triggering international outrage.