Pakistani Shia Muslims search the wreckage of a destroyed bus after a bomb attack on the Pakistan-Iran highway on Jan. 21, 2014. BANARAS KHAN/AFP/Getty

At least 22 Shia pilgrims, many of them women and children, were killed Tuesday in a bomb attack on their bus in western Pakistan.

The bomb exploded near the bus packed with passengers returning from Iran to their home city of Quetta in Balochistan, officials said.

At least 20 people were wounded, said Shafqat Anwar Shawani, assistant police commissioner for Mastung district.

A local government official told the AFP news agency that 51 passengers had been on board at the time of the blast and that several were still unaccounted for.

The provincial home secretary, Asad Gilani, said that two buses had been traveling with government security vehicles and that one of the buses was hit.

Shahwani said bomb disposal officials suspect the bomb was planted on the roadside and detonated remotely, but he did not rule out the possibility of a suicide attack.

He added that the driver of the bus told authorities that he did not see any car hitting his vehicle, but felt a sudden blast followed by the cries of women and children.

Sectarian attacks are on the rise in Pakistan, where minority Shia Muslims make up about 20 percent of the 180-million population.

Human Rights Watch says more than 400 Shias were killed in 2013. They included many professors, doctors and children shot on their way to work or school. The violence is worst in Balochistan province.

Groups such as Lashkar-e-Jhangvi say they are fighting for a Sunni theocracy and that Shias should leave the country or be killed.

No group has claimed responsibility for the latest attack.

Wire services