The jihadists launched an attack at dawn on a checkpoint manned by fighters close to the government of President Abedrabbo Mansour Hadi, the sources said.
They captured three of them before slitting their throats and going on to seize several government buildings.
Al-Qaeda militants have controlled several neighbourhoods of the town in Abyan province for several weeks, according to the military officials.
The Sunni Muslim extremist group has shored up its presence in Abyan where it already controls the provincial capital Zinjibar and the towns of Jaar and Shaqra.
Hadi's internationally recognised government declared Aden the country's provisional capital in March, after Shiite Huthi rebels drove it out of Sanaa and much of northern Yemen.
Al-Qaeda and the Islamic State group have stepped up attacks on Aden despite the government's efforts to secure the city.