TEHRAN, Young Journalists Club (YJC) - PM Ahmed bin Daghar (PM of the ex Yemeni government) and members of his cabinet are believed to be holed up inside the presidential palace in Aden.
There are reports of talks between the southern separatists and ex-government forces, who were previously allies.
The fighting opens up a new front in Yemen, splitting the alliance against Houthis in the north.
It has already led to the death of 40 people since Sunday, the Red Cross says.
The separatists are also reported to have also seized Aden's military bases.
Yemen's ex-government relocated to Aden in 2015, when the resigned President Abdrabbuh Mansour Hadi and his cabinet were forced to flee the capital, Sanaa, following an offensive by the Houthis.
Source: Al-Alam, agencies