The Yemeni government and STC have failed to meet a deadline to establish a power-sharing government, an ominous sign for hopes of a wider deal to end years of war.

Yemen's conflict, which has killed tens of thousands of people and driven millions to the brink of famine, erupted in 2014 when the government was forced out of the capital Sanaa by Iran-aligned Huthi rebels, triggering a Saudi-led military intervention.

In August trouble re-erupted on a separate front, as STC forces seized control of the city of Aden, the internationally recognised government's temporary capital.

In a bid to end the "civil war within a war", Saudi Arabia brokered a power-sharing deal with the Southern Transitional Council (STC) under which the government would return to Aden.

The Riyadh Agreement signed on November 5 also stipulated the creation within 30 days of a new 24-member cabinet with equal representation for the southerners.

Yemen's prime minister Maeen Abdulmalik returned to the city last month but the new cabinet has yet to materialise, along with other key reforms including integrating secessionists into a central command structure.

- 'Very ambitious' -

"The timeline of the Riyadh Agreement was always very ambitious. It is no surprise to see deadlines slip," Elisabeth Kendall, Yemen expert and senior research fellow at Oxford University, told AFP.

"The bigger question is: are the promises simply being delayed, or are they ultimately not achievable?"

The two sides say they are committed to the Riyadh Agreement but have traded accusations over who is responsible for the failure to meet the deadline to form a new government.

STC spokesman Nizar Haitham said on Thursday that Yemen's government was "deviating" from the agreement and mobilising its troops in the south, an accusation the government denied.

However in recent days an STC official told AFP that work to implement the Riyadh Agreement was ongoing and that there has been "significant progress" in implementing military and security arrangements.

"Starting next week, we'll begin steps in implementing what was agreed on," this source said in a written statement without elaborating.

According to government spokesman Rajih Badi, the secessionists are the ones failing to abide by the treaty.

He said that government military movements in the south are in line with the Riyadh Agreement and in coordination with the Saudi coalition, which continues to lead the anti-Huthi camp comprised of both government and secessionist forces.

But although clashes in the south have largely subsided, the situation on the ground remains fragile, said Kendall.

"This is a classic case of an agreement being easy to sign but near impossible to implement," she said.

Other parts of the deal, including placing forces from both sides under the authority of the defence and interior ministries, have also not been fulfilled.

- Overwhelming disappointment -

The unrest in the south distracted the Saudi-led military coalition from its battle against the Huthi rebels.

The lack of concrete progress since the deal was signed comes as a blow to those hailing it as a stepping stone towards ending the wider conflict, described by the UN as the world's worst humanitarian crisis.

"The Riyadh Agreement erected a whole set of deadlines that, for starters, rely on very different Yemeni parties being wholly sincere in wanting and being able to share power in Aden," Neil Partrick, a London-based Middle East analyst, told AFP.

"It's hugely ambitious just to hope to get a power-sharing deal to meaningfully hold up in Aden," he continued.

"But to see this as a basis for the sharing of power throughout the south and for then taking on the Huthis in the north, is possibly not even that serious."

Mohammed Bawzeer, an Aden resident, told AFP there was an overwhelming feeling of disappointment in the city.

"Deadlines have passed, and there is no change on the ground," he said. "We just see things getting worse and worse."