Yemen declared a state of emergency Tuesday and demanded that the UAE forces leave the country within 24 hours, a call backed by Saudi Arabia.
Riyadh warned that its national security was a red line, the strongest language against Abu Dhabi yet, just hours after a Saudi-led coalition carried out an airstrike on the southern Yemeni port of Mukalla, against alleged foreign military support to UAE-backed southern separatists.
Meanwhile, Yemen's presidential council head, Rashad al-Alimi, cancelled a defense pact with the UAE, the Yemeni state news agency said and accused the UAE in a televised speech of fueling internal strife in Yemen with its support to the Southern Transitional Council (STC).
The bombing followed tensions over the advance of STC, which issued a statement supporting the UAE's presence.
Abu Dhabi, in its part, called for "restraint and wisdom" and disputed Riyadh's allegations against it. It did not say whether it would withdraw.
The confrontation threatened to open a new front in Yemen's decadelong war, with forces allied against the Iranian-backed Houthi rebels possibly turning their sights on each other in the Arab world's poorest nation.
It further strained ties between Saudi Arabia and the UAE, neighbors on the Arabian Peninsula that increasingly have competed over economic issues and regional politics, particularly in the Red Sea area. Tuesday's airstrikes and ultimatum appeared to be their most serious confrontation in decades.
"I expect a calibrated escalation from both sides. The UAE-backed Southern Transitional Council is likely to respond by consolidating control," said Mohammed al-Basha, a Yemen expert and founder of the Basha Report, a risk advisory firm.
"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."
A military statement carried by the state-run Saudi Press Agency announced the strikes on Mukalla, which it said came after ships arrived there from Fujairah in the UAE.
"The ships' crew had 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," the statement 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 clear if there were any casualties.
The Emirati Foreign Ministry, hours later, denied it shipped weapons but acknowledged it sent the vehicles "for use by the UAE forces operating in Yemen."
It also claimed Saudi Arabia knew about the shipment ahead of time. The UAE broadly withdrew its forces from Yemen years earlier.
The ministry called for "the highest levels of coordination, restraint and wisdom, taking into account the existing security challenges and threats."
Yemen's anti-Houthi forces declared a state of emergency Tuesday and ended their cooperation with the UAE. They issued a 72-hour ban on border crossings in territory they hold, as well as entries to airports and seaports, except those allowed by Saudi Arabia.
The Southern Transitional Council's AIC satellite news channel aired footage of the strike's aftermath but avoided showing damage to the armored vehicles.
"This unjustified escalation against ports and civilian infrastructure will only strengthen popular demands for decisive action and the declaration of a South Arabian state," the channel said.
The attack likely targeted a ship identified as the Greenland, a 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.
Mukalla is in Yemen's Hadramout governorate, which the council 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 after the rebels seized the capital, Sanaa, in 2014.
Yemen, on the southern edge of the Arabian Peninsula off East Africa, borders the Red Sea and the Gulf of Aden. The war there has killed more than 150,000 people, including fighters and civilians, and created one of the world's worst humanitarian disasters.
The Houthis, meanwhile, have launched attacks on hundreds of ships in the Red Sea corridor over the Israel-Hamas war in Gaza, disrupting regional shipping.
Tuesday's 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 anti-Houthi coalition.
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 to support political forces calling for South Yemen to secede again.
A statement Tuesday from Saudi Arabia's Foreign Ministry directly linked the council's advance to the Emiratis for the first time.
"The kingdom notes that the steps taken by the sisterly United Arab Emirates are extremely dangerous," it said.
Allies of the council later issued a statement in which they showed no sign of backing down.
