A Saudi-led coalition launched an attack in Yemen on Sunday (August 11) - but not against the Houthi movement, who they've been fighting for the past four years.
This time it was against their partners in the conflict.
On Saturday (August 10), separatists known as the Southern Transitional Council effectively took over the port city of Aden, seat of the internationally recognized government.
The separatists, backed by the United Arab Emirates, have been a key part of the coalition fighting the Houthis but they also have a rival agenda to the Saudi-backed government of Abd Rabbu Mansour Hadi - splitting from the north.
On Saturday, they seized control of all government military camps in Aden.
On Sunday, the Saudi-led coalition said it had attacked an area that posed a "direct threat" to Hadi's government.
It did not specify the site, but a local official said separatist forces around the nearly empty presidential palace had been targeted.
Saudi state TV said it was "only the first operation" and that the Southern Transitional Council "still has a chance to withdraw".
The UAE, which has armed and trained thousands of southern separatists, urged calm, while Riyadh said it would host an emergency meeting aimed at restoring order and Hadi's government urged Abu Dhabi to stop backing the southern forces.
The violence complicates the United Nations' efforts to end the Yemen conflict that has killed tens of thousands and pushed the country to the brink of famine.
The UN said on Sunday that as many as 40 have been killed, and 260 injured in Aden since this round of fighting first broke out on Thursday (August 8).