Former UN Secretary-General Kofi Annan is quitting as international peace envoy for Syria in the face of an armed rebellion against President Bashar Assad whose violence shows no sign of abating after 17 months of strife.
As battles raged on Thursday in Syria's second city Aleppo between rebel fighters and government forces using war planes and artillery, UN Secretary Ban Ki-moon announced in New York that Annan had said he would go at the end of the month.
"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," Ban said. Talks were under way to find a successor.
Annan's mission, centered on an April ceasefire that never took hold, has looked irrelevant as fighting has intensified in Damascus and Aleppo.
A clearly frustrated Annan blamed "finger pointing and name calling" at the UN Security Council for his decision to quit.
The White House said Kofi Annan's resignation highlighted President Assad's refusal to abide by a UN-backed peace plan and the failure of Russia and China to hold Assad accountable at the UN Security Council.
"President Assad, despite his promise to abide by the Kofi Annan plan, continues to brutally murder his own people," White House spokesman Jay Carney told reporters aboard Air Force One on Thursday.
Carney also said the White House continues to believe that "Assad must go" and said it did not believe adding more weapons to the Syrian conflict would lead to a peaceful transition.
U.S. ambassador Susan Rice said of the resignation that, "His mission could never have succeeded so long as the Assad regime continuously broke its pledges to implement the Six Point Plan and persisted in using horrific violence against its own people."
In Syria, the fight for Aleppo, the latest battlefield, intensified. Rebels turned the gun of a captured tank against government forces on Thursday, shelling a military airbase used by war planes in the battle for Aleppo.
President Bashar Assad's troops meanwhile bombarded the strategic Salaheddine district in Aleppo itself with tank and artillery fire supported by combat aircraft while rebels tried to consolidate their hold on areas they have seized.
In the capital Damascus, troops overran a suburb on Wednesday and killed at least 35 people, mostly unarmed civilians, residents and activist organisations said.
The fighting for Syria's two biggest cities highlights the country's rapid slide into full-scale civil war 17 months on from the peaceful street protests that marked the start of the anti-Assad uprising.
World powers have watched with mounting concern as diplomatic efforts to find a negotiated solution have faltered and violence that has already claimed an estimated 18,000 lives worsens.
About 60 people were killed in Syria on Thursday, 43 of them civilians, according to the opposition Syrian Observatory for Human Rights.
The rebels' morale was boosted when they turned a government tank's gun on the Menakh airfield 35 km north of Aleppo - a possible staging post for army reinforcements and a base for war planes and helicopter gunships.
"We hit the airport using a tank that we captured from the Assad army. We attacked the airport a few times but we have decided to retreat at this time," a rebel fighter named Abu Ali told Reuters.
Other rebel sources said they had pulled back after coming under fire from MiG warplanes from the airport.
The lightly armed insurgents are battling a well-equipped army that has overwhelming superiority on paper. But the rebels have managed to capture some tanks and heavy weapons and their ranks are swelled by army defectors.
Rebel fighters said they had used improvised explosives in an attack on the Nejrab international airport on the outskirts of Aleppo but there were no reports of serious damage.
Reuters correspondents heard heavy weapons fire on Thursday morning from Salaheddine in southwest Aleppo, a gateway to the city of 2.5 million people that has been fought over for the past week.
Heavily armed government troops are trying to drive a force of a few thousand rebel fighters from the city in battle whose outcome could be a turning point in the conflict.
Although government forces have made concerted efforts to take Salaheddine, a full-out assault on the city as a whole has yet to take place.
Mobile phone connections have been cut since Wednesday evening, leading to speculation among residents that an increase in military action might be imminent.The rebels are consolidating areas they control in Aleppo, attacking police posts and minor military installations with some success. They claim to have seized three police stations this week.
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