A bomb has exploded under a bridge in Pakistan -- shortly before the prime minister, Nawaz Sharif, was due to travel over it.

Pakistan's Information Minister, Mushahid Hussain, described the attack as an act of terrorism directed against the prime minister, but he blamed no group.

The attack, which killed three people and left several others injured, was on the outskirts of the city of Lahore, only a few kilometres from Mr Sharif's family estate.

Police had been checking security along the road when the explosion happened; they say they're still investigating who was responsible.