The flare-up, in Abyan province, comes after the pro-independence Southern Transitional Council (STC) partially withdrew from key sites it occupied in Aden, and a Saudi-led military coalition -- which backs 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 in Zinjibar, the capital of Abyan, and a military camp at Al-Kawd, according to government sources.

Yemeni Information Minister Moammer al-Eryani said in a tweet that the Zinjibar base had been besieged.

"The Security Belt Forces... are demanding the (government) troops surrender or they will storm the camp," he said, referring to the United Arab Emirates-trained force that is aligned with the STC.

STC spokesman Nizar Haitham confirmed to AFP that the two pro-government camps were surrounded by separatist troops.

The Security Belt Forces are "pursuing (agents) working to destabilise security and stability and hiding in the camps belonging to the legitimate government", he told AFP.

The spike in tensions between the separatists and pro-government forces constrains their cooperation against a common foe -- the Iran-aligned Huthi rebels -- in a war that has pushed the country to the brink of famine.

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

They agreed to a withdrawal under pressure by Saudi Arabia and the UAE -- both key to the military coalition supporting the Yemeni government against the Huthis -- but retain control of key military sites.

Yemen's Deputy Foreign Minister Mohammad al-Hadhrami said the latest flare-up will 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 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.

South Yemen was a separate state until it merged with the north in 1990.

Four years later, an armed secession bid 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 crisis.

The UAE is Saudi Arabia's main partner in the coalition fighting the Huthis, but trained and equipped the separatists.

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."