Syrian rebels seized a key northern air base on Tuesday in a new advance after they overran villages in Latakia province, heartland of President Bashar al-Assad's Alawite sect.
The rebel successes came after a string of battlefield setbacks in recent months at the hands of regime forces in the central city of Homs and the nearby town of Qusayr.
"The Islamic State of Iraq and the Levant (ISIS, an Al-Qaeda front organisation) and other opposition groups took total control at dawn today of Minnigh air base" in Aleppo province, the Syrian Observatory for Human Rights said.
State television cited a source it did not identify as saying: "The airport in question is empty of all military apparatus and planes, and is not in use."
Shortly afterwards, it said: "The heroes of our armed forces in Minnigh airport and nearby areas are resisting the terrorists."
Analysts say rebels are trying to clear the north of remaining regime positions, even as the army is attempting to drive them out of the country's centre.
This would presage a possible division of Syria into rebel and regime-held areas, while the Kurds are trying to set up an autonomous area in parts of the north.
The Britain-based Observatory and analysts said advances in the north, and in the coastal province of Latakia, were largely a result of cooperation between jihadist and other rebel groups, often at loggerheads despite sharing the goal of toppling Assad.
"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.
Lister described the base's capture as a "morale boost" for jihadists and for rebels as a whole.
Minnigh lies north of Aleppo city, near the Turkish border.
As the battle entered its final hours, regime forces launched air attacks on several rebel-held towns in Aleppo's countryside, the Observatory said.
And army shelling of a market in Aleppo city's opposition-held Sukkari neighbourhood killed at least five people, including two children, the Observatory said.
"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.
Elsewhere, at least 13 fighters of Al-Nusra Front, another Al-Qaeda affiliated rebel group, were killed in a counter-attack on an arms depot seized by rebels two days ago in Qalamun near Damascus, the Observatory said.
And rebels pressed an advance in the mountains of Latakia, the watchdog added.
In what Observatory director Rami Abdel Rahman described as a "clear advance," local fighters and the two jihadist factions overran 11 villages in three days.
A security source played down the rebel gains, saying only two villages remain in rebel hands after the army counter-attacked.
The fighting is concentrated in the mountainous Jabal al-Akrad and Jabal Turkmen areas, home to a mixed population of Sunnis and Alawites.
Meanwhile, Rome said that Father Paolo Dall'Oglio, an Italian Jesuit who had hoped to negotiate with the jihadists, has "apparently been kidnapped" in Syria.
Dall'Oglio had reportedly gone to meet with ISIS commanders last week to try to negotiate peace between Kurds and jihadists and plead for the release of activists kidnapped by the group.
In other developments, troops raided the northern Deir Baalbeh neighbourhood of Homs and detained some 100 people, who were "taken to an unknown location," the Observatory said.
The arrests came a week after the rebel district of Khaldiyeh fell to the army.
Opposition Syrian National Coalition member Louay Safi condemned what he described as the massacre of more than 200 people in July during the Muslim fasting month of Ramadan.
A Syrian security source said Al-Nusra's chief for Damascus and two deputies had been arrested over attacks in the capital, including one that killed five people and wounded Interior Minister Mohammad Ibrahim al-Shaar and 23 other people in December.
And the UN High Commissioner for Refugees said Syrians who flee their country face recruitment as child soldiers, sexual violence and exploitation for labour, with lawlessness in camps driving exiles home.