The jihadist group did, however, propose a deal to allow the safe passage of critically ill civilians to areas controlled by the US-backed forces in exchange for allowing wounded IS fighters to leave the city, the spokesman said.
The Syria Democratic Forces alliance, made up of Arab and Kurdish fighters and backed by the United States, made rapid advances against the jihadists last month after launching a campaign to flush them out of territory along the Turkish border, through which they had for years moved weapons and fighters.
The SDF quickly surrounded Manbij, but the fight to take the city has been tougher, with slow advances inside it in recent weeks.