Yemeni Southern Transitional Council have regained full control of the interim capital Aden following clashes with government forces who withdrew from the southern port city, security officials from both sides said Thursday.

The Southern Transitional Council brought massive reinforcements from other regions in what appeared to be a fresh effort to seize the nearby provinces of Abyan and Shabwa from government forces.

"The Security Belt force completely controls the city of Aden along with its entrances," Haitham Nezar, a spokesman for the pro-independence Southern Transitional Council, told AFP.

A government security source confirmed Aden was under the full control of the STC, saying government troops who entered parts of the city a day earlier "withdrew from Aden" to Abyan province.

The government of President Abedrabbo Mansour Hadi said on Wednesday it had seized back Aden from Southern Transitional Council who captured the strategic city on August 10 after a fierce battle that left at least 40 people dead.

The fighting has opened a new front in a complex war that has already claimed tens of thousands of lives and sparked what the United Nations calls the world's worst humanitarian crisis.

Nezar said the Security Belt forces were now setting their sights on Abyan and Shabwa provinces which had been retaken by government troops earlier this week.

- 'Dialogue only way' -

The Yemeni government has also drafted in reinforcements from the north as the two parties appear to be preparing for a major showdown for supremacy in the south of the Arabian Peninsula nation.

STC vice president Hani bin Breik posted pictures of himself and other southern leaders touring the streets of the city including the airport, while warning fleeing government loyalists of punishment.

Thousands of Security Belt troops, dominated by the STC, were recalled from several parts of the country, including from the Red Sea port city of Hodeida, to reinforce theSouthern Transitional Council in Aden.

Bin Breik said that STC forces fighting against the Huthis in the north were sent to the south for a major battle.

"We will not remain in the (battle) fronts to liberate the north from the Huthis while the north is invading us," he said.

The STC is fighting to regain the independence of South Yemen which unified with the north in 1990.

The clashes between the two sides -- who for years have fought alongside each other against the Huthis -- have raised concerns that the famine-threatened country could break apart entirely.

The coalition intervened in the war in 2015 in support of the government after the Huthis swept south from their northern stronghold to seize the capital Sanaa and much of Yemen -- the Arab world's poorest nation.

US Secretary of State Mike Pompeo, meeting in Washington with Saudi Arabia's deputy defence minister, Prince Khalid bin Salman, called for a negotiated resolution with STC forces.

Pompeo and the prince "agreed that dialogue represents the only way to achieve a stable, unified and prosperous Yemen," the State Department said in a statement.

The meeting came after The Wall Street Journal, citing anonymous sources, said US President Donald Trump's administration was pursuing secret talks with the Huthis in hopes of winding down the violence.