Unknown gunmen riding a motorbike in Yemen's southern city of Aden assassinated a security officer, a security source said on Wednesday.

Lt. Nasser Abdulqadir Al-Fadhli was shot dead in the district of Al-Mansora late on Tuesday, the source said.

Al-Fadhli graduated from the Saudi King Fahad Security College two years ago. The motives for the crime have not been determined.

Aden, the interim capital of the internationally recognised government, was seized by the UAE-backed southern separatists two years ago.

It has seen hundreds of assassinations of military and security members, politicians, activists and citizens in the past years.

The authorities have failed to address the insecurity and the collapse of basic services including water and electricity.

 -File photo