Former U.N. Secretary-General Kofi Annan is quitting as international peace envoy for Syria in the face of an armed rebellion against President Bashar al-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, U.N. 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 U.N. Security Council for his decision to quit.
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 al-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 organizations 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 (22 miles) 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.
NEW ATROCITIES ALLEGED
In Damascus, still a government stronghold but a scene of combat in the past two weeks, government troops faced new accusations of atrocities after they overran a suburb on Wednesday.
"When the streets were clear we found the bodies of at least 35 men," a resident, who gave his name as Fares, said by phone from Jdeidet Artouz, southwest of Damascus.
"Almost all of them were executed with bullets to their face, head and neck in homes, gardens and basements."
Syrian state television said "dozens of terrorists and mercenaries surrendered or were killed" when the army raided Jdeidet Artouz and its surrounding farmlands.
In a rallying cry to his troops on Wednesday, Assad said their battle against rebels would decide Syria's fate.
But his call to arms, in a written statement, gave no clues to his whereabouts two weeks after a bomb attack on his inner circle.
Assad, who succeeded his late father Hafez 11 years ago to perpetuate the family's rule of Syria, has not spoken in public since the bombing in Damascus killed four of his close security aides, although he has appeared in recorded clips on television.
His low public profile has fuelled speculation about his grip on power since the attack in which his brother-in-law died.
FOOD RUNNING SHORT
The fighting in Salaheddine district, part of a rebel-held arc stretching to the northeast of Aleppo, has left neither side in full control.
On al-Sharqeya Street, residents and shop owners looked in awe at the damage. Some searched through what was left of their buildings - huge piles of concrete and twisted metal.
"I saw death before my eyes," said Abu Ahmed as he abandoned his home. "I was hiding in the alleyway of my building when I heard the whiz of the artillery. Look at my street now."
They said the damage was caused by helicopter fire targeting a rebel brigade based in a school. It missed the school and hit the residential buildings instead.
"This dog Assad and his men are so blind they can't even target a brigade properly," said Abu Ahmed, waving a plastic bag with his meager belongings inside.
State television said on Wednesday the army was pursuing remaining "terrorists" in one Aleppo district and had killed several, including foreign Arab fighters.
Some foreign fighters, including militant Islamists, have joined the battle against Assad, who accuses outside powers of financing and arming the insurgents.
Aleppo had long stayed aloof from the uprising but many of its 2.5 million residents are now caught up in battle zones, facing shortages of food, fuel, water and cooking gas. Thousands have fled and hospitals and makeshift clinics can barely cope with casualties after more than a week of combat.
"The humanitarian situation is deteriorating in Aleppo and food needs are growing rapidly," said the World Food Programme, announcing plans to send emergency food supplies for up 28,000 people.
The U.N. Food and Agriculture Organisation said up to three million Syrians are likely to need food and other aid in the next 12 months because the conflict has prevented farmers from harvesting their crops.
In New York, the U.N. General Assembly was expected to vote on Thursday on a resolution drafted by Saudi Arabia, which is openly supporting the rebel forces seeking to oust Assad.
Russia, which has consistently supported Syria at the United Nations, said it would not back the resolution because the document was unbalanced and would encourage rebels to keep fighting the government.