Kofi Annan has resigned as UN-Arab League envoy for Syria, complaining that his April peace plan had not received the support it deserved from major powers.
As the Syrian army deployed fighter jets against rebels armed with tanks around the commercial capital Aleppo, the former UN chief regretted an "increasing militarisation" of the 17-month conflict.
He also hit out at "continuous finger-pointing and name-calling" at the UN Security Council which he said had prevented coordinated action to end the bloodshed.
Annan said the bickering had hindered his attempts to implement his six-point peace plan which was supposed to start with a reciprocal ceasefire from April 12 that never took hold, but his resignation sparked a new round of recriminations amongst major powers.
"I did not receive all the support that the cause deserved," Annan told a hastily scheduled press conference in Geneva Thursday after his resignation was announced by UN chief Ban Ki-moon at UN headquarters in New York.
"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," he said.
But he 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".
"These crises are never static... as the situation evolves there may be other approaches," he said.
Writing in the Financial Times, Annan called on Russia and the United States to shoulder responsibility for saving Syria from catastrophic civil war.
Annan 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 Putin and Obama," he wrote.
Despite Annan's criticism of the "finger-pointing" at the United Nations, Washington was quick to blame Annan's resignation on the vetoing by Beijing and Moscow of three separate Arab- or Western-drafted resolutions on the Syrian 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.
US secretary of state Hillary Clinton said Annan had "worked tirelessly to try to build consensus in the international community, end the bloodshed, and usher in a government that would meet the legitimate aspirations of the Syrian people".
"Unfortunately, the Security Council was blocked from giving him key tools to advance his efforts," Clinton said in a statement, adding she had thanked him for taking on the "heavy task" of trying to forge a peaceful transition.
Germany also said Annan's decision was partly due to China and Russia's opposition to sanctions on the Damascus regime.
But Russia's envoy to the world body, Vitaly Churkin, insisted Moscow had supported Annan "very strongly", and President Vladimir 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," Russian news agencies quoted Putin as saying on a visit to London for the Olympics, his first in seven years.
China said it regretted the resignation and said it would "work for a political resolution" to the conflict.
"China expresses regret at Annan's resignation. We understand the difficulty of Annan's mediation work, and respect his decision," the foreign ministry said in a statement.
'Most difficult of assignments'
In New York, Ban paid tribute to Annan for his efforts in the "most difficult" of assignments.
"Kofi Annan deserves our profound admiration for the selfless way in which he has put his formidable skills and prestige to this most difficult and potentially thankless of assignments," the UN chief said.
Ban said that the search for a successor had already begun, in cooperation with the Arab League, and European Union foreign affairs chief Catherine Ashton said she hoped it would be completed as quickly as possible.
She said the EU "calls for the early appointment of a successor to carry on Mr Annan's work", adding that his peace blueprint "remains the best hope for the people of Syria".
The Syrian foreign ministry expressed "regrets" at Annan's announcement and accused "countries which seek to destabilise Syria" of having "hindered and continuing to hinder" his mission.
Annan's announcement came as rebel forces shelled the Menagh air base, outside Aleppo, in an intensifying battle for control of Syria's commercial capital.
The rebels used tanks for the first time in its assault on the base, 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.
Salameh was referring to a strategic military checkpoint outside Aleppo that the rebels captured on Monday, opening up a corridor to rear bases in Turkey.
An AFP reporter who witnessed 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".
It is difficult to get an overall picture of the situation inside Aleppo itself because of a lack of independent sources and restrictions on journalists.
Thursday's air base assault came after US President Barack Obama was reported to have signed a covert document authorising US support for the rebels.
The directive was contained in a "finding" -- a device authorising clandestine action by the Central Intelligence Agency, NBC and CNN said, citing unidentified sources.
But the White House spokesman insisted Washington was sticking to its position of sending non-lethal assistance only.
"We don't believe that adding to the number of weapons in Syria is what's needed to help bring about a peaceful transition," Carney said.
The Britain-based Syrian Observatory for Human Rights said a security forces raid southwest of Damascus killed 43 people, some of whom were tortured and executed.
"Regime forces entered the Jdaidet Artuz district on Wednesday and arrested around 100 young people who were taken to a school and tortured," it said.
"On Thursday morning after the operation the bodies of 43 people were recovered. Some of them had been summarily executed."
Nationwide, at least 121 people were killed on Thursday, the majority civilians, said the Observatory, which has estimated that more than 20,000 people have died since mid-March last year.