Saudi Arabia bombed Yemen’s port city of Mukalla on Tuesday over what it described as a shipment of weapons for a separatist force there that arrived from the United Arab Emirates.

The attack signals a new escalation in tensions between the kingdom and the separatist forces of the Southern Transitional Council, which is backed by the Emirates. It also further strains ties between Riyadh and Abu Dhabi, which had been backing competing sides in Yemen’s decade-long war against the Iranian-backed Houthi rebels amid a moment of unease across the wider Red Sea region.

A military statement carried by the state-run Saudi Press Agency announced the strikes, which it said came after ships arrived there from Fujairah, a port city on the UAE’s eastern coast.