Gunmen have killed the deputy director of the main prison in the southern Yemeni city of Taiz and three other people in an ambush on Monday, security official said.

Police sources said gunmen attacked a car carrying Saleh al-Awlaki's car and another vehicle carrying his guards as it passed through a marketplace in the city, killing his daughter and two bodyguards as well before fleeing.

The security official said it was not known who the attackers were. He spoke on condition of anonymity according to security regulations.

Yemen has seen repeated attacks on security personnel since the beginning of a popular uprising against then President Ali Abdullah Saleh last year which split the military into rival factions loyal and opposed to Saleh.

Under a Gulf Co-operation Council-brokered deal, Saleh's deputy took his powers and was later elected as president in a single-candidate poll in February, vowing to fight the al Qaeda-linked Islamists who took control of parts of southern Yemen during the uprising.

A Yemeni military offensive backed by the United States - which feared chaos in Yemen would embolden the al Qaeda branch that planned attacks abroad from the country - drove Islamists from southern strongholds last month.

But a suicide bombing that killed at least 10 people outside a police academy in the Yemeni capital Sanaa has exposed the government's vulnerability to continued attacks by al Qaeda in the Arabian Peninsula despite that campaign.

Jailed al-Qaeda members have escaped in a string of prison breaks dating back to the last years of Saleh's rule, fuelling charges by his foes that he strengthened the hand of Islamists in order to secure his position as a cornerstone of US "counter-terrorism" strategy.