Yemeni security officials say the country's prime minister is preparing to flee after separatists seized the presidential palace in the southern port city of Aden in fierce battles overnight.

The officials say fighters loyal to the so-called Southern Transitional Council fought all way to the gates of the Palace of Maashiq — Arabic for "lovers" — in the district of Crater in Aden. The palace is the seat of Yemen's internationally backed government.

They say Prime Minister Ahmed Obaid Bin Daghar is to leave the country imminently. The officials spoke on condition of anonymity under regulations.

The fighting in Aden first erupted on Sunday, when a deadline issued by the separatists for the government to resign expired.

President Abed Rabbo Mansour Hadi has described the separatists' action as a "coup."