BEIRUT — Syrian rebels captured part of a large army base outside the embattled northern city of Aleppo, tightening the opposition's grip on areas close to the Turkish border, activists said Monday.
The gains by rebel forces came as the European Union denounced the Syrian conflict, which activists say has killed more than 40,000 people.
"The current situation in Syria is a stain on the world's conscience and the international community has a moral duty to address it," European Commission President Jose Manuel Barroso said in Oslo as the EU received the Nobel Peace Prize.
The rebels also killed 13 soldiers in an ambush near a strategic northern town along a road linking the capital, Damascus, with Aleppo, the nation's largest city and its business hub. They also captured 20 soldiers and policemen at a major post on the highway linking the central town of Salamiyeh with the northern city of Raqqa, activists said.
Once on the defensive, Syria's rebels have gained momentum in recent weeks with a number of tactical advances, seizing air bases near Damascus and Aleppo and putting President Bashar Assad's forces on their heels.
In an interview with Dubai TV, Syria's top military defector said Mr. Assad's regime is "over" and advised the president to leave office and let the country's people decide their own fate.
"We are at a turning point and the train of the revolution will be victorious," said Gen. Manaf Tlass, the first member of Mr. Assad's inner circle to break ranks and join the opposition.
Gen. Tlass, who defected in July, said he urged Mr. Assad to listen to the people's demands and implement serious reforms.
"I used to talk to the president four times a day, and I used to see him every other day. I tried to convince him to react with the rebels. He always avoided answering and used to say they are armed gangs," Gen. Tlass said from Paris, where he has been spending much of his time.
"I told him tens of times, and sometimes in a loud voice that 'you should be with your people' and he did not answer," Gen. Tlass said. "It's over. I advise him to leave."
The Britain-based Syrian Observatory for Human Rights said the rebels entered the Sheik Suleiman military base outside Aleppo on Sunday afternoon after weeks of fighting around the facility.
Last month, they captured another base near the city, the Syrian army's 46th Regiment base.
Rami Abdul-Rahman, who heads the Observatory, said the rebels who stormed the Sheik Suleiman base belong to hard-line Islamic militant groups.
Mr. Abdul-Rahman told The Associated Press that fighters from Jabhat al-Nusra, Mujahedeen Shura Council and the Muhajireen group took part in the battle for the base.
The Islamic militant groups, which count Syrians and foreigners in their ranks, are among the most effective fighters on the rebel side of the country's civil war.
But the West is wary of such groups, and U.S. officials have said the Obama administration is preparing to designate the Jabhat al-Nusra group, which has alleged ties to al Qaeda, as a terrorist group.
The Observatory said the rebels seized key sectors of the base, home to the 111th Regiment, including its command center.
About 140 Syrian troops fled to a nearby base as the rebels advanced, Mr. Abdul-Rahman said, adding that opposition fighters captured seven government troops and killed two soldiers in the fighting.