Fresh fighting flared in Yemeninfo-icon's southern city of Aden on Monday after separatist forces seized government buildings in what the prime minister said was an attempted coup.

Aden has served as the headquarters of Saudi-backed president Abd-rabbu Mansour Hadi's government since it was forced out of the capital Sanaa by Houthi fighters three years ago, AFP reported.

The separatists--who want the return of the independent South Yemen that existed before 1990--supported Hadi's forces against the Houthis but tensions between the two sides have risen in recent months.

The flare-up in Aden has added yet another dimension to one of the worldinfo-icon's most complicated conflicts, a civil warinfo-icon that left thousands dead and millions on the brink of starvation.

On Monday, sporadic clashes continued after fighting overnight in the port city, especially in its north where separatist forces tried to take control of a military camp, security sources said.

The separatists dispatched additional forces from the central province of Marib and the southern province of Abyan, the sources said.

The forces from Abyan progressed towards Aden after clashes with loyalists on the way.

After the separatists seized the government headquarters on Sunday, Prime Minister Ahmed bin Dagher on Sunday denounced a "coup... in Aden against legitimacy and the country's unity".

He urged a Saudi-led military coalition backing Hadi to intervene in its defense. The coalition launched air strikes against the Houthi fighters in March 2015 and sent troops to support Hadi's forces.

  Shooting All Night

On Sunday, security sources said pro-separatist units trained and backed by the United Arab Emiratesinfo-icon had taken over the government headquarters in Aden after clashes.

By early evening, separatists took control of two roads leading to the presidential palace where several members of the government were staying, security sources said.

The International Committee of the Red Cross said fighting continued overnight in the port city.

"All night shooting in Aden #Yemen, including heavy weaponsinfo-icon," Alexandre Faite, the head of the ICRC delegation in the country based in Sanaa, said on Twitterinfo-icon.

"Those in southern part of city, including [ICRC staff] still unable to get out."

UN envoy to Yemen Ismail Ould Cheikh Ahmed urged all parties to return to "calm and dialogue".

The top negotiator is to step down in April after three years of overseeing UN-brokered negotiations between the government and rebels, none of which have stemmed the violence in the country.

More than 13000 people have been killed in Yemen since the USinfo-icon-backed Saudi-led military coalition intervened.

The separatists joined forces with Hadi's government to oust the Houthis from southern provinces in 2015, but tensions have soared since a secessionist governor's sacking last year.

  Outlaws

Dagher held a cabinet meeting overnight to discuss "military developments and sabotage acts targeting government installations", loyalist news agency Saba reported.

He condemned the actions of "outlaws" against the "legitimacy represented by President Hadi" in the city.

Sunday's fighting in Aden killed 15 people including three civilians, hospital sources said, after separatist protesters were prevented from entering the city for a rally to demand the government's ouster.

The separatists accused the prime minister of ordering his troops to open fire at the protesters.

Sunday's rally was called by the Southern Transitional Council, an autonomous body not recognized by the government and aimed at overseeing self-governance in southern provinces.

Former Aden governor Aidarous al-Zoubeidi formed the council in May last year after Hadi fired him.

The council had asked Hadi to make changes in the government and gave him one week to do so--a deadline that expired on Sunday. It had warned that if Hadi did not accept the demand, its supporters would begin a protest campaign to oust Dagher's government.

South Yemen was independent--with former British colony Aden as its capital--from its formation in 1967 until 1990, when it was unified with North Yemen.