Saudi Arabia declared a UAE-backed separatist advance in Yemen a threat to the kingdom’s national security on Tuesday and called Abu Dhabi’s actions “highly dangerous”, as the rivalry between the Gulf monarchies boiled over into an open dispute.

The strongly worded foreign ministry statement came hours after the Saudi-led coalition fighting in Yemen said it had struck a United Arab Emirates weapons shipment destined for separatist forces.

“The steps taken by the UAE are considered highly dangerous,” it said, adding “the Kingdom stresses that any threat to its national security is a red line, and the Kingdom will not hesitate to take all necessary steps and measures to confront and neutralise any such threat”.

The United Arab Emirates later rebuffed the accusations from Saudi Arabia, saying: “the shipment in question did not contain any weapons, and the vehicles unloaded were not intended for any Yemeni party.”

In recent weeks, separatists backed by the UAE seeking to revive the formerly independent state of South Yemen have made sweeping territorial gains in a lightning offensive.

Experts say their successes have embarrassed Saudi Arabia, a regional heavyweight and the main backer of the internationally recognised Yemeni government.

“Given the danger and escalation posed by these weapons… the coalition air forces carried out a limited military operation this morning targeting weapons and combat vehicles that had been unloaded from the two ships at the port of Al-Mukalla,” the alliance said, according to the Saudi state news agency.