Opposition activists say fighting between U.S.-backed Syrian rebels and rival militants has killed more than 20 people over the past two days.

The fighting between the so-called Democratic Forces of Syria, which is led by Kurds, and Islamic militants has flared in recent days in the northern province of Aleppo.

The Britain-based Syrian Observatory for Human Rights says the fighting is concentrated near the town of Azaz, close to the border with Turkey. Azaz has been repeatedly struck by Russian airstrikes in recent days.

The Observatory says 15 militants and eight DFS fighters have been killed since Sunday.

Bahaa al-Halaby, an opposition activist based in Aleppo, said Monday that 20 civilians were killed in the fighting but had no figures of fighters killed.