Islamist militants and fighters loyal to the Syrian government have made a rare prisoner swap, with insurgents releasing 25 women and children in exchange for one of their commanders, a monitoring group said.

The Syrian Observatory for Human Rights said 10 children and 15 women were kidnapped by insurgents more than a year ago from Shi'ite towns in northern Aleppo province.

The deal was between the militant Islamist Jaish al-Mujahideen and pro-government militia, it said, and mediated by Kurdish fighters known as Popular Protection Units (YPG).

Syrian state media did not mention the exchange, and government officials were not immediately available for comment on Monday.

"The women and children were kidnapped between a year and 18 months ago and held by different rebels," Observatory head Rami Abdulrahman told Reuters by phone. "They were handed to Jaish al-Mujahideen to use in an exchange."

He said the commander was the military leader of Jaish al-Mujahideen, who was captured in August.

Kidnap for ransom is common in Syria but trades between opposing sides in the fighting are infrequent. More than 220,000 people have been killed in Syria's conflict.

The Observatory said the women and children were from Nubl and Zahra, two Shi'ite villages that have been under a long siege by anti-government forces in an isolated Shi'ite area.