BEIRUT — Rebels captured part of a massive Syrian 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 rebels also killed 13 soldiers in an ambush near a strategic northern town along a road linking Aleppo, the nation’s largest city and business hub, with Damascus, and captured 20 soldiers and police officers at a major post on the highway linking the central town of Salamiyeh with the northern city of Raqqa, activists said.
In Washington, the Obama administration declared a Syrian rebel group with alleged ties to Al Qaeda as a terrorist organization.
The State Department action, which was included in the Federal Register on Monday, blocks Jabhat al-Nusra’s assets in the United States and bars Americans from doing business with the group. The move is an effort to blunt the influence of extremists as the United States steps up cooperation with the Syrian opposition.
The department said Jabhat al-Nusra is part of Al Qaeda in Iraq. The group has claimed responsibility for suicide bombings on Syrian government targets, raising fears of growing Islamic extremism among the opposition.
Secretary of State Hillary Rodham Clinton will be traveling to North Africa this week to try to bolster moderates hoping to end President Bashar Assad’s regime in Syria.
Monday’s gains by rebel forces in northern Syria 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.
Once on the defensive, Syria’s rebels have gained momentum in recent weeks with a number of tactical advances, seizing airbases near Damascus and Aleppo and putting Assad’s forces on their heels.
In an interview with Dubai TV, Syria’s top military defector said Assad’s regime is over and he advised the president to leave office and let the country’s people decide their own fate.
Manaf Tlass, a Syrian general who was the first member of Assad’s inner circle to break ranks and join the opposition, said, ‘‘We are at a turning point and the train of the revolution will be victorious.’’
Tlass, who defected in July, said he urged 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,’’ 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,’’ Tlass said. ‘‘It’s over . . . I advise him to leave.’’
The fighting has also spilled repeatedly into the neighboring countries of Turkey, Lebanon, and Jordan. Clashes between Sunni Muslim and Alawite militias in Tripoli, Lebanon, over the last week killed at least 17 people.
Tripoli, which is Lebanon’s second-largest city and is close to the northern border with Syria, has long been the scene of conflict between Sunni Muslims and Alawites, and both groups have militias.
Many Lebanese Sunnis have supported the Sunni-led uprising against Assad of Syria, who is Alawite and whose Muslim sect dominates the government.
The Britain-based Syrian Observatory for Human Rights said Syrian rebel forces 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 Sheik Suleiman belong to hard-line Islamic militant groups such as Jabhat al-Nusra, Mujahedeen Shura Council, and the Muhajireen group.
The groups, which count both Syrians and foreigners in their ranks, are among the most effective fighters on the rebel side of the country’s civil war.