Southern separatists seized two Yemeni government military bases near the southern port of Aden early on Tuesday.
The attacks triggered fresh clashes between nominal allies that have complicated UN peace efforts.
The separatists and government are both part of a Saudi-led military coalition battling the Houthi movement, which took over the capital Sanaa and most major cities in 2014.
On another front in the north, the Saudi-led coalition said it launched airstrikes overnight on Houthi military targets in Sanaa, Reuters reports. Caves storing missiles, drones and weapons were struck, according to the coalition.