An opposition activist group and a rebel say Kurdish fighters and their allies have captured a military air base in northern Syria.

Abdul-Jabbar Abu Thabet, a local rebel commander in the Aleppo province, said Thursday that Mannagh air base fell to the People's Protection Units, or YPG, and their allies after fierce battles.

The Britain-based Syrian Observatory for Human Rights said the offensive came as warplanes believed to be Russian carried out 30 airstrikes in the area. It said the air base and a nearby village, also called Mannagh, fell late Wednesday.

With Syrian troops backed by Russian warplanes waging a major offensive between the northern city of Aleppo and the Turkish border, the Kurds appeared to be exploiting the chaos to expand their nearby enclave, known as Afrin.