Air strikes by the US-led coalition killed at least 21 civilians in and around a stronghold of the Islamic State group in northern Syria on Monday, a monitor said.

At least 15 civilians were killed in raids in a northern district of Manbij while six others were killed in a village near the city, according to the Syrian Observatory for Human Rights.

The militant bastion has faced a more than month-long US-backed offensive by Kurdish and Arab fighters that has caused thousands of civilians to flee.