Yemeni security officials say military forces are trying to retake areas held by Islamist militants in the southern port city of Aden, as clashes between the two sides intensify.

The officials say the Saturday clashes in Aden's Mansoura district have expanded southward as civilians in the area fled their homes.

Local affiliates of both al-Qaida and the Islamist State are fighting against security forces, with the fighters using heavy weapons in guerrilla warfare. A Saudi-led coalition started bombing Islamist locations in the city for the first time, the officials said.

They spoke on condition of anonymity because they were not authorized to brief reporters.

Al-Qaida and the Islamic State have exploited the chaos of Yemen's 18-month civil war to expand the territory under their control in southern and eastern Yemen.