Clashes raged in the Yemeni city of Aden on Saturday between members of the separatist Southern Transitional Council (STC) and at least four fighters have been killed, two security sources said.

Residents said heavy gunfire could still be heard at mid-day in Aden's central Crater district that houses government headquarters and the central bank. The port city has seen tension between the internationally recognised government and the STC over control of the south of the country.

The prime minister of the Saudi-backed government returned to Aden last week from Saudi Arabia and is residing at the presidential palace in Crater along with other government ministers. Yemen's president is based in Riyadh.

"We ask citizens in Crater to remain at home during the next few hours as security and anti-terrorism forces clean the city from some...outlaw elements," the STC's Security Belt Forces unit said in a Twitter post.