At least 19 Shiite Muslims were killed when a car bomb detonated near their bus in south-western Pakistan on Sunday, a senior administration official and witnesses said.
The blast occurred in Mastung district of Balochistan province, which has seen numerous attacks on the minority Muslim sect.
Tufail Baloch, deputy commissioner of Mastung, said a three-bus convoy of Shiite pilgrims was travelling under escort when a pickup truck parked by the roadside exploded and destroyed the first passenger vehicle.
"Nineteen people died and up to 25 others, including four women, sustained injuries."
Baloch said all passengers in the remaining two buses escaped unhurt, but at least two security officers in an escort vehicle sustained injuries.
It was not immediately clear whether someone was present in the explosives-laden pickup truck, or if it was detonated by a remote control device.
The buses were headed towards the Taftan border crossing with Iran, located 590 kilometres west of the bombing site. Thousands of Shiite Muslims regularly visit Iran for religious tourism.
Survivors were returned to the provincial capital, Quetta, after authorities stopped them from continuing with their journey.
In Balochistan, Shiites have been a frequent target of attacks, mostly blamed on radical Sunni groups that also have links with Islamist insurgents.
Separately, a roadside bomb killed two Pakistani soldiers in the North Waziristan tribal region on Sunday, security sources said.
No group claimed responsibility for the attack, but the mountainous territory along the Afghan border is a hotbed of Taliban and al-Qaeda insurgents.