Syrian opposition chief Ahmad Moaz al-Khatib is determined to step down in the face of what he sees as world "inaction" as government forces recapture territory from the rebels, colleagues said on Sunday.
Troops loyal to President Bashar al-Assad made advances outside the capital and near the Lebanese border, and a watchdog said that scores of bodies, some disfigured, had been recovered after days of ferocious fighting.
Khatib's renewed threat to quit, almost a month after he first tendered his resignation amid recriminations over the choice of a perceived Muslim Brotherhood nominee as rebel prime minister, threw the opposition into fresh disarray as debate about the wisdom of arming it intensifies.
"I can confirm Khatib's resignation is final," National Coalition member Marwan Hajjo told AFP, following a meeting in Istanbul of the "Friends of Syria" group of Arab and Western governments that support the opposition.
A source close to the Coalition told AFP that its members have already launched consultations in the search for a successor.
Khatib himself posted a short statement on his Facebook page saying: "When a bird is in his cage, he remains imprisoned and paralysed. Yesterday I came out of the cage of deception that I was in."
Hajjo, who heads the Coalition's membership committee, said Khatib was stepping down because of a "lack of real action on behalf of the Syrian people."
"The international community, the Friends of Syria group, should be providing heavy weapons to enable the Syrians to defend themselves," he said.
In Istanbul, the United States pledged to double its aid to the armed opposition, including additional non-lethal military equipment.
But it again held back from agreeing to opposition calls to arm the rebels, amid mounting concerns that weapons deliveries might fall into the wrong hands in a conflict in which Al-Qaeda has played a prominent role on the battlefield.
Assad loyalists made advances on Sunday around the rebel-held town of Qusayr near the Lebanese border.
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Opposition activists said the army was backed by militiamen and fighters of the Lebanese Shiite movement Hezbollah.
"Loyalist troops backed by Hezbollah have taken control of important villages near Qusayr," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
Four shells fired from the Qusayr area struck the Hermel region of eastern Lebanon, in what rebels described as retaliation for Hezbollah's intervention on the side of the Assad government.
Lebanon's National News Agency reported that three shells hit early on Sunday, while a security official and a resident of the town of Hermel said a fourth struck later in the day.
Qusayr has been under rebel control for more than a year.
"The only reason why the regime is advancing in the Qusayr area is because of Hezbollah's troops. Hezbollah fighters advance on the ground, while the (Syrian) air force gives them cover," Qusayr-based activist Hadi al-Abdallah said via the Internet.
In a meeting with a visiting Lebanese delegation on Sunday, Assad paid tribute to the "Syrian people's resistance and support for the courageous army," the official SANA news agency reported.
Outside Damascus, the army stormed the town of Jdaidet al-Fadl, after five days of fighting in which at least 80 people have been killed, the Observatory said.
Abdel Rahman told AFP he feared the death toll could be higher but that it was difficult to document after the entry of regime forces into the town.
"We call on the International Committee of the Red Cross to send a delegation to Jdaidet al-Fadl in order to investigate," he said.
The Observatory distributed amateur video footage that it said was filmed in the town and that showed the bodies of men laid out on the ground, some with mutilated faces and covered in blood.