Rebels seized control of a key air base in Aleppo province Tuesday, marking another major advance after overrunning a string of villages in Latakia province, the heartland of President Bashar al-Assad's Alawite sect.
The rebel successes come after a string of battlefield setbacks in recent months at the hands of regime forces in the central city of Homs and in the town of Qusayr in Homs province.
"The Islamic State of Iraq and the Levant (jihadist group) and other opposition groups took total control at dawn today of Minnigh air base," said the Syrian Observatory for Human Rights.
Syrian state television only gave a veiled recognition of the rebel assault and said loyalist troops were fighting on.
It first cited an unnamed media source as saying: "The airport in question is empty of all military apparatus and planes, and is not in use."
Shortly afterwards, the broadcaster said "the heroes of our armed forces in Minnigh airport and nearby areas are resisting the terrorists".
Analysts say the rebels are trying to clear northern Syria of remaining regime positions, even as the army is attempting to drive the opposition out of the country's centre.
This presages a possible de facto division of Syria into rebel and regime-held areas, with the Kurds also trying to set up an autonomous area in parts of the north, according to analysts.
The Britain-based Observatory and analysts said the progress in Latakia and Aleppo was largely thanks to cooperation between jihadist and local rebel groups, who have often been at loggerheads despite sharing the goal of toppling Assad.
The takeover comes a day after the rebels on Monday launched a fresh assault spearheaded by a Saudi jihadist who "blew himself up in an armoured vehicle at the entrance to the headquarters of the Minnigh air base," said the group.
"The victory again underlines the leading strategic impact being played by militant Islamists, particularly in northern Syria," said London-based Charles Lister of IHS Jane's Terrorism and Insurgency Centre.
The analyst described the capture of the base as a "morale boost" for jihadists and for Syria's rebels as a whole.
Minnigh is located north of Aleppo city, near the Turkish border.
As the battle for Minnigh entered its final hours, regime forces launched air attacks on several rebel-held towns in Aleppo's countryside, the Observatory said.
In Aleppo city, army shelling on a souk (market) in the opposition-held Sukkari neighbourhood left at least five dead, among them two children and two women, said the Observatory.
"These were revenge attacks... The only means the regime has at its disposal is the shelling of civilian targets from far away," said Yehya Naanaa, who heads Aleppo province's opposition council.
Despite the violence, the rebel takeover of Minnigh "lifted the spirits" of people in Aleppo, Naanaa said via the Internet.
Elsewhere, at least 13 Al-Nusra Front jihadists were killed in a regime counter-attack on an arms depot seized two days ago by Liwa al-Islam and other rebel groups in Qalamun near Damascus, said the Observatory.
Rebels meanwhile pressed an advance in the mountains of Latakia in coastal Syria, according to the Observatory.
In what the Observatory director Rami Abdel Rahman described as a "clear advance", Latakia's local rebels alongside the jihadist Al-Nusra Front and the Islamic State of Iraq and the Levant in three days overran 11 villages.
A security source downplayed the advance there, however, saying that only two villages remain in rebel hands after the army counter-attacked.
"The reason the rebels advanced was because members of the pro-regime militia who were surveying the area were bribed to leave" their hilltop position, the source told AFP.
The fighting is concentrated in the mountainous Jabal al-Akrad and Jabal Turkman areas, which are home to a mixed population of Sunnis and Alawites.
The Alawite sect is an offshoot of Shiite Islam, and Assad hails from the Latakia town of Qardaha.
Italy's foreign ministry meanwhile said Father Paolo Dall'Oglio, an Italian Jesuit priest who had hoped to negotiate with jihadists in Syria, has "apparently been kidnapped".
Known for his opposition to the Assad regime, Dall'Oglio had reportedly gone to meet with commanders of the Islamic State of Iraq and the Levant to try to negotiate peace between Kurds and jihadists and to plead for the release of activists kidnapped by the group.