BEIRUT - Rebels captured part of a sprawling 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.

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 President Bashar al-Assad's forces on their heels.

In an interview with Dubai TV, Syria's top military defector said Assad's regime is "over" and 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."

The Britain-based Syrian Observatory for Human Rights said the rebels entered the Sheikh Suleiman military base outside Aleppo on Sunday afternoon, after weeks of fighting around the facility.

Rami Abdul-Rahman, who heads to Observatory, said the rebels who stormed the base belong to hard-line Islamic militant groups such as Jabhat al-Nusra, Mujahedeen Shura Council, and the Muhajireen group.

The groups 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 the United States has designated the Jabhat al-Nusra group, which has alleged ties to al-Qaeda, a terrorist organization.

The move, which blocks Jabhat al-Nusra's assets in the U.S. and bars Americans from doing business with the group, was included in the Federal Register on Monday.