The security official said Saturday’s explosion took place on the road linking Aden to Zinjibar, the provincial capital of Abyan. Zinjibar has fallen under the control of Islamic militants affiliated with Al-Qaeda.

Militants have seized a number of cities in southern Yemen, taking advantage of political turmoil that erupted in February with mass protests against longtime President Ali Abdullah Saleh.

The official said gunmen stormed the checkpoint after Saturday’s bombing.

The official was speaking on condition of anonymity because he was not authorized to speak to the media.