DUBAI (Reuters) - The Saudi-led coalition fighting the Houthi movement in Yemen blocked southern separatist leaders from returning to Aden city, Yemeni officials said on Thursday, as a deal to end a power struggle in the south with the Saudi-backed government faltered.

The separatists are part of a Sunni Muslim coalition that intervened in Yemen in March 2015 to try to restore the government of President Abd-Rabbu Mansour Hadi, which was ousted from the capital Sanaa in 2014 by the Iran-aligned Houthis.

However last year, the separatist Southern Transitional Council (STC), which is backed by the United Arab Emirates and seeks self-determination in the south, turned on Hadi's government and seized its interim capital in Aden, sparking wider clashes that opened a new front in the war.

As part of a Saudi-brokered peace deal reached last year in Riyadh to end the stand-off between the Yemeni government and southern separatists, Saudi forces have taken control of most of south Yemen, following a withdrawal of the UAE military.

But the deal's implementation, including the formation of a new government, has stalled and tensions have been rising between Saudi-backed forces and STC fighters.

"Your southern resistance has been following closely the news about banning several southern leaders from returning to Aden," the STC said in a statement on Thursday.

"This sets a dangerous precedent that could trigger a popular uprising in the south which would be devastating," the statement added.

At least five STC leaders, including Aden's security chief Shalal Ali Shayea, were about to board a flight from Amman to Aden, sources familiar with the situation told Reuters.

The airplane could not take off after the Saudi-led coalition denied it clearance to enter Yemeni airspace, they said.

Coalition spokesman Colonel Turki al-Malki said no in-air flight was prevented from landing in Aden.

The kingdom's foreign ministry said it was committed to the implementation of the Riyadh agreement and called on both sides to work together and avoid escalation.

The move followed weeks of tensions between the two sides competing for control and influence.

Hadi's government had asked the UAE to stop arming and training separatist forces, while Abu Dhabi has said Hadi's government is ineffective and that it distrusts Islamists with whom the president is allied.

Riyadh began informal talks with the Houthis in late September on decreasing hostilities.

But fierce battles resumed last month and culminated with the Houthi group taking control of its capital al-Hazem.

The recent escalation has shattered more than three months of calm in the five-year-old conflict.