The opening of a new front in a complex and devastating war prompted the UN's Yemen envoy to warn that the country could splinter unless a peace deal is quickly reached.

The latest clashes in Abyan province came after the pro-independence Southern Transitional Council (STC) partially withdrew from key sites it occupied in Aden earlier this month.

A Saudi-led military coalition backing the government said it had "succeeded in calming the situation".

But on Tuesday, fighters from the so-called Security Belt Forces surrounded a special forces camp near provincial capital Zinjibar.

The site is about 60 kilometres (37 miles) from Aden, and close to Al-Kawd military camp.

Abyan governor Abu Bakr Hussein told AFP the separatists then seized the Al-Kawd camp in fierce clashes, forcing out the 350 troops there.

They remained positioned around the Zinjibar base following the exit of government forces, in a deal mediated by local authorities.

At least four military personnel -- two separatists and two government troops -- were killed and 23 wounded in the fighting, said Hussein, adding that 1,100 troops had been stationed in Zinjibar.

Mohammed al-Markhi, a Security Belt Forces commander, confirmed his troops had control of both camps, while Zinjibar residents said separatists were also deployed in the city's streets.

The separatists have received Emirati support and training, despite the United Arab Emirates being a key pillar in the Saudi-led coalition backing the government against Huthi rebels.

While they have also fought against the Huthis, STC forces want to see South Yemen regain the independence it gave up with unification in 1990.

- 'No time to lose' -

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The war against the Iran-aligned Huthis has already pushed the country to the brink of famine.

The UN's special envoy to Yemen, Martin Griffiths, condemned the STC's takeover of Aden and told the Security Council that there was "no time to lose" in brokering a peace deal.

"We certainly cannot underestimate the risks that these events pose for the future of the country," he said via video from Jordan.

"No country can tolerate the stresses of internal conflict indefinitely."

"The stakes are becoming too high for the future of Yemen, the Yemeni people and the wider region," he added.

The STC announced late Tuesday that its head Aidarus al-Zubaidi had left for the Saudi city of Jeddah on the invitation of the Saudi foreign ministry.

The Security Belt Forces ousted supporters of President Abedrabbo Mansour Hadi from what was the capital of the formerly independent south on August 10 in clashes that left around 40 people dead.

They had agreed to a partial withdrawal under pressure by Saudi Arabia and the UAE, but retain control of key military sites.

- 'Unjustified escalation' -

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Yemen's Deputy Foreign Minister Mohammad al-Hadhrami said the latest flare-up would undermine peace talks.

"What Abyan governorate is witnessing is an unjustified escalation by the STC," the Yemeni foreign ministry quoted him as saying.

"It is something that is rejected and unacceptable and will undermine mediation efforts by Saudi Arabia."

"We reject the continued provision of financial and military support by the UAE to outlawed STC forces in Yemen," he added.

In an interview with pan-Arab daily Asharq al-Awsat, STC spokesman Nizar Haitham said the group was open to dialogue but ruled out any withdrawal from the military posts in Aden.

"There will be no dialogue if we were to hand over all the positions... what will there be left to negotiate," he said in remarks published on Tuesday.

An armed secession bid four years after Yemen's unification ended in occupation by northern forces, giving rise to resentments which persist to this day.

The Saudi-led military coalition sent a delegation to Aden -- the Hadi government's base since the Huthi rebels took over Yemen's capital Sanaa in 2014 -- on Thursday to discuss the new front in the country's crisis.

Analysts say the break between Hadi's internationally recognised government and the separatists reflects a wider rift between Riyadh and Abu Dhabi.

The UN's Yemen envoy Martin Griffiths said he held a "positive and engaging" meeting with Saudi's deputy defence minister, Prince Khaled bin Salman, on Monday to discuss the crisis.

"Tireless role under Khaled bin Salman's leadership to restore order and stability in south Yemen," he tweeted Tuesday.

"We agreed on the need for continuous dialogue."