Yemeni separatists have taken effective control of the port city of Aden after days of fighting with troops loyal to the internationally backed government, BBC reports.
Forces aligned with the UAE-backed Southern Transitional Council (STC) - which wants an independent south - said they had seized control of military camps and the presidential palace.
The government said it was a "coup".
The action has fractured the Saudi-led coalition against Houthi rebels that both sides have fought in.
The coalition called for an immediate ceasefire in the city and said it would "use military force against anyone who violates it", Saudi Arabia's state news agency SPA reported.
The separatists agreed to the ceasefire, which was due to come into effect early on Sunday.