The Saudi-led coalition in Yemen said on Thursday that the United Arab Emirates had spirited a separatist leader out of the country in a dramatic twist to a row between the Gulf powers, as witnesses said Saudi-backed forces advanced to the port of Aden after losing ground there.

The Saudi-led coalition said Aidarous al-Zubaidi, head of a UAE-backed southern separatist group, had left Yemen by boat for Somaliland, before boarding an aircraft to Mogadishu that was later tracked to a military airport in Abu Dhabi.

The move escalates tensions between Saudi Arabia and the UAE, major global oil heavyweights and close allies of the United States. Zubaidi had failed to show up in Riyadh for crisis talks over turmoil in southern Yemen on Wednesday.

Zubaidi's separatist group, the Southern Transitional Council (STC), said he had been asked to go to Saudi Arabia under threat.

The Saudi claim that the UAE helped him escape raises the stakes in a crisis that erupted last month when the separatists swept through southern Yemen and reached the border with Saudi Arabia, which declared the move a threat to its national security.

The fast-moving developments caused a rift between Saudi Arabia and the UAE, fracturing a coalition headed by Yemen's internationally recognised government which is battling the Iran-backed Houthis.

The two most powerful countries in the Gulf have sharp differences over a wide range of volatile issues across the Middle East — from geopolitics to oil output — which burst out into the open with the crisis in Yemen.

Brazen escape

After al-Zubaidi's unexplained absence from the Riyadh talks, his group said he was overseeing military and security operations in the southern port city of Aden.

Aden had been the main seat of power in Yemen outside Houthi-controlled areas since 2015, but leaders of the Saudi-backed government left the city for Saudi Arabia when the STC took control last month.

On Thursday, witnesses and Yemeni government officials said Aden was coming under the control of Saudi-backed forces, which have also been advancing in other areas.

Reuters could not verify the situation in Aden, and there was no Saudi confirmation that its coalition was in control there.

In a sign that a split may be emerging among the separatists, an STC delegation said it had held fruitful talks in Riyadh with the Saudi ambassador to Yemen, according to comments by Mohammed Al Ghaith, a senior official of the group.

Any such fracture could complicate efforts to stabilise Yemen's south.

If Zubaidi's presence in the UAE capital is confirmed, it could anger the Saudis, who pressured the UAE to rein in the separatists after their advance through south Yemen.

In a statement on Thursday, the coalition said Zubaidi and others accompanying him on the plane to Mogadishu from Somaliland were under the supervision of UAE officers and waited an hour before flying to a military airport in Abu Dhabi.

The coalition statement also mentioned by name the UAE officer whose help Zubaidi had sought.

The aircraft was of a type similar to those frequently used in conflict zones on the routes of countries such as Ethiopia, Libya and Somalia, the coalition added.

The plane turned off its identification system over the Gulf of Oman, before turning it back on 10 minutes prior to arrival at Al Reef military airport in Abu Dhabi, the coalition said.

The coalition did not clearly say if Zubaidi was still aboard en route to the UAE capital.

There was no immediate comment from the UAE or Zubaidi's STC.

UAE's assertive foreign policy

The UAE has pursued an assertive foreign policy and carved out its own sphere of influence across the Middle East and Africa, a strategy thrown into the spotlight after the rare military escalation with Saudi Arabia in Yemen.

It withdrew its remaining forces from Yemen and has called for de-escalation in the country, reeling from one of the world's worst humanitarian crises caused by the civil war.

Saudi Arabia and the UAE first intervened in Yemen after the Houthis seized the Yemeni capital of Sanaa in 2014.

The UAE joined the Saudi-backed coalition the following year in support of the internationally recognised government.

The STC was set up in 2017 with UAE backing and eventually joined the government coalition.