At least 13 people have been killed in a bomb explosion on a passenger bus in Pakistan's troubled south-western province of Balochistan, officials say.
About 20 others were injured in the blast which happened as the bus was travelling through Kolpur town.
Balochistan, the source of Pakistan's main gas reserves, has been the scene of rising violence between rebels and security forces.
The rebels are demanding greater control over natural resources.
Government officials said they were investigating whether a bomb had been deliberately left on the bus, or if an explosive device being carried by militants had gone off prematurely.
The bus had been travelling from the provincial capital, Quetta, to the eastern city of Lahore.
Interior minister Aftab Sherpao said the bus was carrying 50 passengers.
Last week, tribesmen blew up a gas pipeline and fired more than 200 rockets at a major base belonging to the Pakistani security forces in the district of Dera Bugti, about 350km (250 miles) from Quetta.
The army launched a major crackdown in Balochistan last month after rockets were fired during a visit by President Pervez Musharraf.