At least 36 people have been killed and 185 wounded in two days of heavy fighting between separatists and government troops in Yemen's interim capital of Aden, the International Committee of the Red Cross said.
Fighting intensified on Tuesday night after the two sides used tank and artillery on the second day of an attempted "coup" in the southern port city, military sources said.
Earlier reports said Yemeni separatists had surrounded the presidential palace in Aden. "The separatists have surrounded the palace and now control the main gate. Those inside are unofficially under house arrest at this point," a high-ranking officer with the Yemeni military told AFP.
Tension between the separatists and the government erupted into clashes at the weekend, with more than 36 people killed in Aden since Sunday, according to the Red Cross.
Brigadier Saleh al-Sayyed, who heads troops that have fought alongside the separatists since Sunday, announced his forces had seized control of the Fourth Brigade, the presidential guard in Aden.
The southern city has served as the government's de facto capital since 2014, when the Huthi rebels seized control of Sanaa in their fight against the state. Saudi Arabia-backed President Abedrabbo Mansour Hadi resides in Riyadh.
The Saudi-led military coalition which supports the government called on Tuesday for an immediate ceasefire and an end to "all forms of armed conflict", in a statement cited by Saudi state news agency SPA.
It said it would take "all necessary measures to restore security and stability" after earlier calls for restraint went unheeded.
The flare-up has added yet another dimension to one of the world's most complicated conflicts, a civil war that has left thousands dead and millions on the brink of starvation.
Formerly independent states, North and South Yemen were unified in 1991 under the presidency of Ali Abdullah Saleh, who was killed in December in clashes with the Huthis in Sanaa.
The Huthi rebels, who are backed by Iran, control most of northern Yemen. The south is home to the Southern Transitional Council, a separatist movement that wants to reinstate South Yemen as an independent entity.
The head of the Red Cross mission in Yemen, Carlos Batallas, said on Twitter on Monday that the clashes in Aden had killed 36 people and wounded 185. The ICRC did not provide figures on civilian casualties.
Residents were hunkered down at home as five separatist fighters were killed by snipers and four soldiers died in clashes, military sources said, with tanks and heavy artillery entering the fray.
The clashes came after separatist forces seized government buildings in what Yemen's prime minister called an attempted coup.
At night, pro-separatist security forces advanced in some areas and were reported just 1km away from the presidential palace in Aden. They also seized two military camps near Aden international airport which remained closed for the second day.
Fighters from both sides have been deployed in most areas of Aden, paralysed for a second day after 15 people were killed and dozens wounded on Sunday.
The hostilities erupted early Sunday when pro-government troops prevented supporters of the separatist Southern Transitional Council from entering Aden for a rally.
Separatists have dispatched additional forces from the central province of Marib and the southern province of Abyan, security sources said.
After the separatists seized the government headquarters on Sunday, Prime Minister Ahmed bin Dagher denounced a "coup ... in Aden against legitimacy and the country's unity".
He urged the Saudi-led coalition to intervene in its defence.