Kurdish fighters backed by Russian bombing raids have driven Syrian rebels from a former military air base near the border with Turkey, a group that monitors the war said on Thursday.
Rebel groups have been distracted by a major offensive in the area by the Syrian army and its Russian and other allies, allowing the Kurds to capture the base and expand their foothold in the north.
The Syrian Observatory for Human Rights, which tracks the war through a network of sources on the ground, reported heavy fighting between Kurds and Syrian rebels around Menagh air base, a former Syrian air force facility that had fallen into rebel hands.
Russian warplanes staged at least 30 raids against rebels at the base before the Kurdish fighters were able to seize it.
One rebel commander, Zekeriya Karsli from the Levant Front, said: "The fall of Menagh airport has made the situation on the ground pretty grim."
Kurds in northern Syria have established a degree of autonomy since the start of the war in some areas bordering Turkey, which is struggling to end a three-decade insurgency on its own territory by Kurdish militants who want more self-rule.
Both Syrian and Turkish Kurds, however, stop short of a declared bid for independent statehood, unlike Iraqi Kurdistan, which is already an autonomous region and is moving toward a referendum to declare full independence from Baghdad.
The Russians are happy to help the Kurds in this instance as it means further problems for the Syrian rebels they are trying to destroy.
Kurdish YPG militias have taken advantage of the rebels' preoccupation with fending off the Russian-backed Syrian army offensive launched last week in the northern Aleppo countryside to gain ground near an important border crossing with Turkey, the Syrian insurgents say.
Kurdish fighters based in the city of Afrin, south of rebel-held Azaz, have taken a series of villages, including Deir Jameal and al-Qamiya, which rebels have been forced to evacuate as Syrian troops advance from the south.
Major offensive
The intensive Russian bombing of rebel towns in northern Aleppo province, while avoiding the Kurds in Afrin, allowed the Kurds to move on Menagh airport, which the rebels had held since August 2013.
"The Kurds have gained from the major offensive in Aleppo to widen their areas of control," Rami Abdulrahman, head of the Observatory, said.
The loss of the air base, which lies near the road between Aleppo and the Turkish city of Gaziantep, reflects the dramatic change in the balance of forces since Russia began its military intervention on Sept. 30 on behalf of President Bashar al-Assad.
Other rebel fighters trapped by Kurdish forces to the west and Syrian army and allied militias advancing from the south, are now seeking to defend Tal Rifaat, heavily hit by Russian bombers in the last two days, the Observatory and a rebel source confirmed.
Syrian troops are only a few kilometers from the town.
Russian bombing had allowed Syrian troops supported by Iranian-backed Shi'ite militias to advance to almost 25 km (15 miles) from the Turkish border, the closest they have come in more than two-and-a-half years.
The Bab al Salam border crossing with Turkey was a main trucking gateway from Europe via Syria to the Gulf before the war. Moscow and Damascus say it is a conduit for arms to the rebels supplied by Turkey.
The Syrian army advance has cut a rebel supply line between the border and the parts of the city of Aleppo which the insurgents control. The army is now seeking to regain full control of what was once the country's most populous city.
The Kurdish campaign to expand in rural areas in northern Aleppo province it considers as ethnically Kurdish has aroused suspicion among mainstream Arab rebels.
"They are trying to advance by exploiting our concentration on fighting the regime to win more territory," Abu Mustapha al-Saleh, a commander from al-Jabha al Shamiya group, said from Azaz.
"On the ground it looks as though they are waging one operation and of course the selectiveness of the Russian bombing confirms this," he added.
The heavy bombing has forced tens of thousands of Syrians to flee to the safety of the border areas around Azaz town. Prevented from entering Turkey, many have also gone to safer areas in the mainly rebel-held north-west province of Idlib.
A spokesman for Turkey's IHH aid organization, Burak Karacaoglu, said: "The Russian strikes are still the single biggest threat to our humanitarian aid work inside Syria."
(Reporting by Suleiman Al-Khalidi and Humeyra Pamuk; editing by Angus MacSwan and Giles Elgood)