Southern separatists gained ground across Yemen's second city Aden on Saturday and surrounded the presidential palace as they fought fierce battles with loyalist forces, military and security sources said.
Deadly fighting raging in Aden since Wednesday is pitting unionist forces loyal to the internationally recognised government against a force that supports it but is dominated by fighters seeking renewed independence for the south.
The force, known as the Security Belt, overran three military barracks belonging to unionist forces and were surrounding the presidential palace, sources close to the Security Belt said.
Fierce clashes were also taking place in several other parts of the city.
An AFP correspondent reported seeing separatist fighters surrounding a tank which they claimed they had seized from a military position.
Southern Yemen was an independent state until 1990 and the north is widely perceived to have imposed unification by force.
The Security Belt is a force trained by the United Arab Emirates, a key partner in a Saudi-led military coalition which intervened in Yemen more than four years ago to prop up Hadi's government in the face of an uprising by Iran-aligned Shiite Huthi rebels.
The Huthis control large parts of northern and western Yemen, including the capital Sanaa.
On Thursday, Hadi's government urged Saudi Arabia and the UAE to put pressure on the Security Belt to avoid a military escalation in Aden.
UAE Foreign Minister Abdullah bin Zayed Al-Nahyan expressed "deep concern" on Saturday over the violence in Aden and called for a "de-escalation".
"Sheikh Abdullah called for a responsible and serious dialogue to end the differences and work on unity in this delicate phase while maintaining security and stability." the official Emirati news agency WAM reported.
He said the UAE was "exerting all efforts to calm and de-escalate the situation in Aden", saying the two camps should focus their efforts on fighting the Huthis not each other.
Sheikh Abdullah also called on the UN envoy to Yemen Martin Griffiths "to make all possible efforts to end the escalation in Aden", the statement added.
Fighting in the city flared on Wednesday after two members of the Security Belt were killed in clashes with other loyalist forces after the funeral of police personnel killed in the city last week, according to security officials.
A missile and drone attack by Huthi rebels on a Security Belt training camp just outside Aden last week killed 49 people, many of them newly graduated cadets.
The UN human rights office this week accused the Security Belt force of "reportedly carrying out and enabling retaliatory attacks against civilians" from northern Yemen.
Griffiths wrote on Twitter on Wednesday that he was "alarmed by the military escalations in Aden".
The fighting has left at least 18 dead and scores of wounded, medics and security sources have said.
On Friday, Doctors Without Borders (MSF) tweeted that it had treated 75 people in one of its hospitals in Aden since Thursday night.