Fighters from Yemen's separatist Southern Transitional Council sit on the back an armoured vehicle in Aden's northern Dar Saad district on January 30, 2018, as they move closer to taking full control of the southern city

Separatists in war-ravaged Yemen have seized all but one district of the government's de facto capital Aden, also laying siege to the isolated presidential palace on the city's southern outskirts on Tuesday.

Global charity Save the Children said it was suspending life-saving work in Aden as its staff were forced to hunker down amid gun battles that the Red Cross says have killed at least 36 people in the past three days.

A number of ministers fled the port city by boat on Monday night, docking in the district of Brega further west, a military source told AFP.

A port official confirmed their arrival, saying they were transported to a base of the Saudi-led coalition which backs Yemen's government.

But the ministers returned to Aden before dawn, after receiving guarantees from the coalition that the presidential palace would not be stormed, a government source said.

The source said Saudi Arabia and its coalition allies are now in talks with southern separatists and with Yemen's government, which Riyadh supports in the war against Iran-backed Huthi rebels.

The separatists, who advocate for self-rule in southern Yemen, now control seven of Aden's eight districts.

A tank in Aden's northern Dar Saad district flies a flag belonging to the separatist Southern Transitional Council as STC forces move closer to taking full control of the southern city on January 30, 2018

Forces loyal to the government of President Abedrabbo Mansour Hadi hold the large northern district of Dar Saad, while ministers remain in the besieged presidential palace near the coast.

Hadi's government has accused the separatists of attempting a coup in Aden, opening yet another front in the country's devastating conflict.