The Assad regime and opposition groups have swapped detainees in northern Syria, Turkey's Foreign Ministry said on Saturday, describing it as a first step to build confidence between the warring sides.

The move was part of a pilot project prepared by a working group formed under the Astana process by Turkey, Russia, Iran and the United Nations to investigate the fate of missing people and release those who have been detained, the ministry said.

It did not specify how many people were involved in the swap but the Syrian Observatory for Human Rights, a U.K.-based war monitoring group, said opposition factions had released 10 hostages in return for the Assad regime releasing 10 detainees.

"Some people who were held in detention in Syria by opposition groups and the regime were set free in the area, under opposition control, of Abu Zindeyn south of al-Bab," said a written statement from the Turkey's ministry.

"The aim is to maintain with new initiatives this practice, which represents an important first step in terms of building confidence between the sides," it added.

Regime's news agency SANA said nine men and one woman "were liberated" after militants held them in the Aleppo countryside. A video on SANA showed them on a bus clapping and cheering.

Separately, regime shelling killed five children and two women in Syria's opposition-held Idlib province, where Russia and Turkey have agreed on a buffer zone, the Observatory said.