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 UAE did not immediately acknowledge the strike.
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 decadelong war against the Iranian-backed Houthi rebels amid a moment of unease across the wider Red Sea region.
Yemen's anti-Houthi forces later declared a state of emergency Tuesday. It issued a 72-hour ban on all border crossings in territory they hold, as well as entries to airports and seaports, except those allowed by Saudi Arabia.
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.
"The ships' crew had the disabled tracking devices aboard the vessels, and unloaded a large amount of weapons and combat vehicles in support of the Southern Transitional Council's forces," it said.
"Considering that the aforementioned weapons constitute an imminent threat, and an escalation that threatens peace and stability, the Coalition Air Force has conducted this morning a limited airstrike that targeted weapons and military vehicles offloaded from the two vessels in Mukalla," it added.
It wasn't immediately clear if there were any casualties from the strike or if any other military besides Saudi Arabia's took part. The Saudi military said it conducted the attack overnight to make sure "no collateral damage occurred."
The UAE did not immediately respond to a request for comment from the AP. The Council's AIC satellite news channel acknowledged the strikes, without offering details.
The attack likely targeted a ship identified by analysts as the Greenland, a roll-on, roll-off vessel flagged out of St. Kitts. Tracking data analyzed by the AP showed the vessel had been in Fujairah on Dec. 22 and arrived in Mukalla on Sunday. The second vessel could not be immediately identified.
Mohammed al-Basha, a Yemen expert and the founder of the Basha Report, a risk advisory firm, cited social media videos which purported to show new armored vehicles rolling through Mukalla after the ship's arrival. The ship's owners, based in Dubai, could not be immediately reached.
"I expect a calibrated escalation from both sides. The UAE-backed Southern Transitional Council is likely to respond by consolidating control," al-Basha said. "At the same time, the flow of weapons from the UAE to the STC is set to be curtailed following the port attack, particularly as Saudi Arabia controls the airspace."
Footage later aired by Saudi state television, which appeared to be filmed by a surveillance aircraft, purportedly showed the armored vehicles moving through Mukalla to a staging area. The types of vehicles corresponded to the social media footage.
Mukalla is in Yemen's Hadramout governorate, which the Council had seized in recent days. The port city is some 480 kilometers (300 miles) northeast of Aden, which has been the seat of power for anti-Houthi forces in Yemen after the rebels seized the capital, Sanaa, back in 2014.
The strike in Mukalla comes after Saudi Arabia targeted the Council in airstrikes Friday that analysts described as a warning for the separatists to halt their advance and leave the governorates of Hadramout and Mahra.
The Council had pushed out forces there affiliated with the Saudi-backed National Shield Forces, another group in the coalition fighting the Houthis.
Those aligned with the Council have increasingly flown the flag of South Yemen, which was a separate country from 1967-1990. Demonstrators have been rallying for days to support political forces calling for South Yemen to secede again from Yemen.
The actions by the separatists have put pressure on the relationship between Saudi Arabia and the UAE, which maintain close relations and are members of the OPEC oil cartel, but also have competed for influence and international business in recent years.
There has also been an escalation of violence in Sudan, another nation on the Red Sea, where the kingdom and the Emirates support opposing forces in that country's ongoing war.
Meanwhile, Israel has acknowledged Somalia's breakaway region of Somaliland as an independent nation, the first to do so in over 30 years. That's sparked concern from the Houthis, who have threatened to attack any Israeli presence in Somaliland.