Explainer-How Yemen's southern power shift heightens risk in turbulent region 

Dec 9 (Reuters) - Yemen's southern separatists have claimed control over swathes of territory in an apparent major shift in power that risks rekindling a 10-year-old civil war ​after a long lull, raising new uncertainties in a country near important sea routes.

The ​separatist Southern Transitional Council (STC), backed in the past by the United Arab Emirates, says it has taken over the eastern provinces of Hadhramaut and Mahra and is now firmly established across ⁠all provinces of the former state of South Yemen.

The STC has been an important part of the coalition fighting alongside the internationally-recognised government against the Houthi movement, which holds Yemen's capital Sanaa and the heavily populated northwest.

WHY HAS YEMEN FALLEN APART?

Yemen, situated between Saudi Arabia and an important shipping route on the Red Sea, ​was split into northern and southern states until 1990.

South Yemen was the only Communist Arab state and agreed unification with the north ‍after a factional civil war in 1986 that wiped out its ​political leadership, and as its main financial patron the Soviet Union collapsed.

After unification the north dominated and the main southern party tried to secede, leading to a brief war in 1994 that was quickly won by the northern government based in Sanaa.

Meanwhile in the north the Houthi group emerged in the late 1990s fighting guerrilla wars against the government over what they saw as marginalisation of their Zaydi Shi'ite sect.

After Arab Spring protests erupted in 2011, Yemen's army fell apart. Gulf countries backed a transition ⁠with an interim government in Sanaa and talks on a new federal constitution that was opposed by both the Houthis and the southern separatists.

The Houthis captured Sanaa in late 2014 and the interim government fled south in 2015 with a Saudi-led coalition intervening on its behalf against the Houthis.

WHY DOES THE STC'S SEIZURE OF TERRITORY MATTER?

Yemen's civil war has been frozen along relatively stable frontlines for years, and the STC's takeover of much of the south now risks upsetting that fragile status quo.

Houthi raids on Red Sea shipping over the Gaza war disrupted global trade, underscoring the potential for renewed conflict reverberating beyond Yemen, though there is no sign of that yet.

The seizure of areas in the east means it now holds much of Yemen's modest oil reserves and production facilities, while the takeover of Aden, the old southern capital, poses problems for the internationally-backed government.

Yemen overlooks the Bab ​el-Mandeb Strait, a narrow waterway connecting the Red Sea ⁠to the Gulf of Aden and a critical maritime trade route between Europe, Asia and the Middle East.

WILL THE SOUTH ⁠SECEDE?

The STC was formed in 2017 from the myriad rival groups that once made up the southern movement.

It describes independence as an aspiration of all southerners and it flies the flag of the old South Yemen state. However, it has also said it seeks autonomy for now, and says its immediate priority is fighting the Houthis.

At the heart of the STC are a network of powerful military and security leaders, closely tied to the UAE, who emerged during a battle to oust the Houthis from Aden in 2015.

It ‌joined Yemen's Presidential Council, the body that took over the duties of head of state in the internationally-backed government, in ​2022. However, it is not clear how far the southern movement has been able to put aside the regional, tribal and ideological splits that plagued it for decades and unite around common goals.

WHAT ABOUT THE INTERNATIONALLY-RECOGNISED GOVERNMENT?

While the internationally-recognised government has formally been based in Aden since fleeing the Houthis in early 2015, it has spent much of that time operating from the Saudi capital Riyadh.

Its head Rashad al-Alimi and Prime Minister Salem Saleh Bin Braik both ‍left Aden for Riyadh when the STC took over, the southern group said.

Alimi said the STC's actions undermine the government's legitimacy and violate power-sharing agreements. But the government has weathered splits in its coalition before, with STC fighters clashing with government loyalists in 2017.

HOW DO THE HOUTHIS FIT IN?

The Houthis remain firmly entrenched across the most populous ‌areas of Yemen in the highlands and Red Sea coast.

While they remain bitter enemies of both the internationally backed government and the STC there has been ‌little fighting for years and it is far from clear whether the southern gains will change that.

WHICH FOREIGN STATES ARE INVOLVED?

The Houthis are aligned with Iran, which has provided them with some training and equipment.

Saudi Arabia has long wielded influence in Yemen and it intervened in 2015 to support the political transition it had brokered and because of concerns about the Houthi relationship with Riyadh's regional arch nemesis Iran.

It backs the internationally-recognised government and various forces which support it, including Muslim Brotherhood-aligned fighters around Marib in the north.

The UAE joined the ⁠Saudi-led coalition in 2015 and was militarily active in the south, where it backed southern groups, revealing frictions between the two Gulf monarchies. The UAE is fiercely opposed to the Muslim Brotherhood.

(Writing by Angus McDowall, Editing by William Maclean)