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 Aleppo, the nation's largest city and business hub, with Damascus, and 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 airbases 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 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." 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 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 to Observatory, said the rebels who stormed Sheik Suleiman belong to hardline 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. But the West is wary of such groups, and the U.S. has designated the Jabhat al-Nusra group, which has alleged ties to al-Qaida, 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, hasn't been announced officially but was included in the Federal Register on Monday. The department said the group was part of al-Qaida in Iraq.
In the fight for the Sheik Suleiman base, as the rebels seized key sectors of the facility, which is home to the 111th Regiment, including its command center, the Observatory said.
About 140 Syrian troops fled to another, nearby base as the rebels advanced, Abdul-Rahman said, adding that opposition fighters captured seven government troops and killed two soldiers in the fighting.
Amateur videos released by activists showed gunmen walking inside the base, carrying a militant black Islamic flag.
The footage also showed rebels driving around in a captured tank and manning heavy anti-aircraft machine guns. The activist videos appear genuine and correspond to AP's reporting on the events depicted.
Fighting around Syria has intensified in the past few months. The uprising, which began with peaceful protests against Assad's regime in March 2011, has escalated into a civil war.
"This is what we captured from Assad's army," a rebel says in the video, carrying an automatic rifle and a walkie-talkie and pointing to the heavy machine guns.
Abdul-Rahman said the rebels tried to storm Sheik Suleiman base two weeks ago but were pushed back by troops who killed nearly two dozen rebel fighters.
The Observatory also reported heavy fighting Monday on the southern edge of the strategic rebel-held town of Maaret al-Numan, captured from government troops in October. It said rebels ambushed an army unit, killing at least 13 soldiers.
The group said Syrian warplanes bombed the town after the death of the solders.
Mohammed Kanaan, an activist based in Maaret al-Numan, said rebels stormed army positions south of the town and killed many soldiers.
"The town is witnessing some of the worst clashes in weeks," said Kanaan via Skype.
Maaret al-Numan is on the highway that links Damascus with Aleppo, Syria's largest city and commercial center that has been the scene of clashes between rebels and troops since July.
Abdul-Rahman said the rebels captured another major post on the highway linking Salamiyeh and Raqqa in what could be a major move to disrupt another route of supplies for the regime. It said nine troops were killed in Monday's fighting.
An amateur video showed more than 15 men inside a room who identified themselves as troops who were posted on the highway in the Syrian desert near the town of Bukraya.
Activists also reported violence in other areas, including the eastern province of Deir el-Zour, central region of Homs as well as villages and towns near Damascus International Airport south of the capital.
A resident in Damascus, speaking on condition of anonymity for fear of government reprisal, told The AP that he was hearing cracks of gunfire and explosions inside and outside the capital early Monday.
The Observatory said troops clashed with rebels in the Damascus neighborhoods of Salhiyeh and Rukneddine north of the city in which three people were killed. It said the clashes were "the most intense in the neighborhoods since the revolution began."
Associated Press writer Bradley Klapper in Washington contributed to this report.