Jihadists led by the radical Al-Nusra Front seized a strategic army base in the northern Syrian province of Aleppo on Monday after weeks of fierce fighting, a watchdog said.
The violence came after UN-Arab League envoy Lakhdar Brahimi reported that he held "constructive" talks in Geneva at the weekend with senior Russian and US officials, and ahead of a Friends of Syria meeting in Morocco.
The capture of the base at Sheikh Suleiman dealt a blow to President Bashar al-Assad's regime in the region as it had been the last major military base west of Aleppo city still under army control.
It also undercut the military influence of the mainstream rebel Free Syrian Army, which was not involved in the takeover, said the Syrian Observatory for Human Rights.
The army used warplanes and tanks, meanwhile, to bombard rebel positions in Damascus province, where the regime is desperate to suppress an insurgency that is inching ever closer to the capital, the monitoring group said.
"Al-Nusra Front, alongside several Islamist rebel battalions linked to it, have seized control of the army base headquarters at Sheikh Suleiman," Observatory director Rami Abdel Rahman told AFP.
Base 111, with its headquarters in the village of Sheikh Suleiman, sprawls over nearly 200 hectares (500 acres) of rocky hills about 25 kilometres (15 miles) from Aleppo, Syria's second city.
"It is a significant win for the rebels. It proves the army is still suffering major military losses," said Abdel Rahman, adding that the rebels seized up to 10 military vehicles and at least one tank.
Other rebel groups also claimed to have captured the base, but Abdel Rahman insisted Al-Nusra and its allies were responsible for its fall.
An AFP journalist who covered the clashes around Sheikh Suleiman said many of the fighters were from other Arab countries and Central Asia.
Elsewhere, the army carried out air strikes on suburbs of the capital, amid fierce clashes in Damascus province, said the Observatory.
"A fighter bomber carried out a raid on Daraya as rebels and soldiers fought on the ground, while artillery bombed Moadamiyet al-Sham," said the Britain-based watchdog, referring to two towns south of Damascus.
Al-Watan newspaper, close to the government, said the army on Sunday "dealt heavy blows to Al-Nusra Front gunmen linked to the Al-Qaeda network in several regions of Damascus province and Aleppo".
Soldiers also attacked gunmen in the Tadamun and Hajar Aswad districts of southern Damascus, "killing or wounding dozens of them", it said.
Al-Watan said government forces had killed more than 5,000 rebels in Aleppo in the past month.
Ninety-four people, mostly civilians, were killed on Sunday across Syria, with the Damascus area accounting for 37 of the deaths, said the Observatory, which relies on activists and medics for its information.
Brahimi, the Algerian troubleshooter, described as "constructive" a meeting on Sunday with Russian and US representatives who stressed "a political process to end the crisis in Syria was necessary and still possible".
The latest violence comes two days ahead of a Friends of Syria nations meeting in Marrakesh, which will bring together countries that support the anti-Assad revolt.
Arab and Western states will consider two key issues concerning the 21-month conflict -- the political transition in the event of Assad's fall, and mobilising vital humanitarian aid as winter sets in.
Since the last meeting, in Paris in July, the number of people killed has risen from 16,000 to more than 42,000, according to the Observatory.