Unidentified gunmen on Wednesday shot dead two policemen in Yemen's southern city of Aden, a security official said, in the second such attack in as many days.

The assailants opened fire at the traffic policemen at a roundabout in the country's second city before escaping on a motorbike, the official told AFP.

There has been a string of killings by jihadists in Aden targeting members of the security forces and the government, which has set up its temporary base there.

Al-Qaida and the rival Islamic State group (IS) both have a foothold in the city, where jihadists occupy government buildings and patrol several districts.

On Tuesday, unidentified gunmen killed a police lieutenant colonel from the airport's investigation unit as he left his home.

Aden was rocked by months of fighting last year between pro-government forces and rebels who seized the capital in September 2014 before expanding southwards.

Loyalists backed by a Saudi-led coalition have wrested back control of Aden and four other southern provinces since July, but the rebels still hold Sanaa and have besieged the third city of Taez for months.