Three people have been killed and four others wounded in Yemen after a bomb blast ripped through a minibus at a central bus station in Sanaa, a police official has said.

The explosion on Thursday took place at the capital's al-Hasha bus station.

"A bomb exploded on a minibus, killing three people and wounding four among the passengers," the AFP news agency quoted an unnamed security official as saying.

The official said that police were investigating the attack.

Some reports disputed that the explosion had been caused by a bomb, saying that the bus had been running on liquified gas.

In recent months Yemen has seen a series of assaults on security services and oil installations claimed by the al-Qaeda group, though these attacks have rarely targeted civilians.

Elsewhere in the capital on Thursday, seven people including two policemen were injured in clashes between security forces and opposition supporters protesting against polls to be held next year.