A banned extremist group claimed responsibility for an attack on a bus carrying Shia Muslims in Pakistan's southwestern Balochistan province on Tuesday, which killed 26 people.

Three more Shia Muslims were killed in Quetta, the provincial capital, when they were going to hospital to collect bodies of those killed in the bus attack.

The outlawed Sunni extremist group "Lashkar-e-Jhangvi" claimed responsibility for both attacks and warned more similar sectarian attacks.

A purported spokesman for the group, Ali Sher Haidary, called journalists in Quetta via phone and claimed responsibility for the attacks.

The bus driver who survived the attack told media that four unidentified gunmen had arrived in a jeep and stopped the bus near Mastung district on its way from Quetta to Iran for pilgrimage and asked all the passengers to get off the bus and to stand in a queue and opened fire on them.

Local sources said that the gunmen continued firing for 10 minutes and then fled after believing that they have killed many of the passengers.

An official in the region said that 26 people died at the scene and some 30 others were injured.

The injured were admitted in a local hospital and the critically injured were shifted to Quetta. Emergency was declared in the Quetta main hospitals, health officials said.

Shia groups strongly condemned the attack and said the government should have taken steps for the security of the passengers in view of the recent sectarian-motivated attacks.

They said they would stage protest rallies against the incident across Pakistan and will press the government to arrest the culprits.

President Asif Ali Zardari has strongly condemned the attack. The President termed the attack as inhuman and a brutal act of terrorism and said such cowardly acts cannot dent the government's resolve to fight this menace to the finish.

He expressed his deep sense of shock and grief over the loss of lives in this brutal act of terrorism and conveyed his sympathies to the families of the victims.