Lashkar-e-Jhangvi spokesman Abu Bakar telephoned journalists in Quetta, the capital of Balochistan province, to claim responsibility for yesterday's attack.
He said the attack was carried out in retaliation for the recent bombing of a madrassa and a Tablighi Jamaat centre in Quetta.
Balochistan Home Secretary Naseebullah Bazai confirmed to the media that the attack was carried out by a suicide bomber.
The attacker rammed his explosives-laden vehicle into the bus that was carrying Shia pilgrims returning from Iran.
Two policemen escorting the pilgrims were among the dead.
Police said they had arrested about two dozen suspects during raids in different parts of Quetta.
Balochistan has become a flashpoint for sectarian and ethnic violence.
In several recent attacks, suspected LeJ militants have stopped buses, separated members of the minority Hazara Shia community from other passengers and gunned them down in cold blood.
Shia groups observed a strike today to mourn those who died in yesterday?s attack and markets across Quetta remained shut.
Responding to a call from the Hazara Democratic Party, businessmen and traders kept their establishments closed.
Contingent of police and Frontier Corps were deployed in sensitive areas of Quetta to maintain law and order.
No incident of violence was reported during the strike.
Islamabad: The banned Lashkar-e-Jhangvi has claimed responsibility for a suicide attack on a bus that killed 13 people, most of them Shia pilgrims returning from Iran, in Pakistan's Quetta city even as police rounded up about two dozen suspects on Friday.
First Published: Friday, June 29, 2012, 22:23