Four killed during fighting between Yemeni separatist forces
At least four fighters were killed, two security sources said.
Residents reported heavy gunfire in Aden's central Crater district that houses government headquarters and the central bank.
The city has seen tension between the internationally recognised government and the STC over control of the south.
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.
South Yemen has been paralysed by the power struggle between the government and the United Arab Emirates-backed STC, leading to protests in recent months over widespread poverty and poor public services.
They are nominal allies under a coalition led by Saudi Arabia which has been battling the Iran-aligned Houthi movement that ousted the government from the capital Sanaa in late 2014 and now holds most of northern Yemen and main urban centres.
Saudi Arabia mediated a deal aimed at ending the standoff between the government and the STC, including forming a new cabinet that includes separatists, but a planned troop redeployment has yet to take place.
The coalition intervened in Yemen in 2015 against the Houthis but the conflict has dragged on, killing tens of thousands and pushing the country to the brink of famine.