Militants in western Pakistan bombed a bus carrying women students on Saturday and then seized part of the hospital where survivors of the attack were taken, killing at least 21 people, officials said.

At least 27 were injured.

The gunmen in Quetta, the capital of Baluchistan province long plagued by sectarian violence, were holed up in the emergency ward of a hospital, engulfed in a firefight pitting them against the security forces.

Security forces had forced their way into part of the Bolan Medical Complex, where dozens of patients and staff were believed to be trapped. Television footage showed troops surrounding the building and a helicopter hovering overhead.

"They are several in number, we are still facing resistance from them, and people are stranded inside the hospital. We are trying our best to rescue the people," said Jan Mohammed Bulaidi, spokesman for Baluchistan's new chief minister, who took office last Sunday.

The attack in resource-rich Baluchistan was Pakistan's most lethal since the government of Prime Minister Nawaz Sharif took office last week and followed earlier explosions in a nearby town that killed a policeman and destroyed a historic building.

Quetta is a hotbed of sectarian violence, much of it targeting the Hazara ethnic minority, who are Shia Muslims.

The province is also racked by a separatist insurgency. It was not immediately clear who was responsible for the bus and hospital attacks, or whether they was aimed at the Hazaras.

City police chief Mir Zubair Mehmood told Reuters that the students on the bus were from various ethnic groups, including Hazaras, targets of a series of bombings this year.

The initial blast gutted the bus while it was on the campus of a local university for women, killing 11 students, and another explosion went off soon after at the hospital, the city's largest. Television footage showed people fleeing the building in panic. Shots were fired from automatic weapons.

Bulaidi said he believed the second attack was targeted at government officials.

Those killed at the hospital included a senior government official, three security officials and a nurse, he said. The city's nursing federation said three more nurses and two family members of the student victims also died.

"The Frontier Corps troops have cleared certain parts of the hospital and they are moving forward," he said.

A doctor trapped inside told the PTV state television network gunfire was continuing. Eight people, he said, had been severely injured in the second blast.

"Eight people have shrapnel injuries," he said.

Saturday's attack was the biggest since bombings in the city at the start of the year killed almost 200 people, briefly drawing global attention to a growing campaign of victimization of the Hazaras by sectarian militants.

The 500,000-strong community in Quetta has been subjected to a campaign of shootings and bombings by Lashkar-e-Jhangvi (LEJ), a militant group dedicated to attacking Pakistan's Shi'ite Muslim minority.

Earlier in the day, suspected separatists killed a policeman and gutted an historic summer retreat used by Pakistan's founder Muhammad Ali Jinnah in a hill town in the province, days after a new government vowed to end a guerrilla war there.

Several men surrounded the heritage Quaid Azam Residency in the hill town of Ziarat in the early hours of the morning, before detonating several explosive devices, local police and an eyewitness said.

A policeman died and the ensuing blaze tore through the two-storey wood-clad building. It was not clear what type of weapon caused the blasts. Earlier, a senior local government official said rocket propelled grenades had hit the building.

Baluchistan borders Iran and Afghanistan. The province has experienced waves of revolt by Baluch nationalists since being incorporated into Pakistan in 1948. Rights groups say security forces have unleashed a campaign of forced disappearances.

Baluchistan's top policeman Mushtaq Sukhera said the one rebel group, the Baluchistan Liberation Army, had claimed responsibility for the attack on the house. Reuters was not able to contact the insurgents to independently verify that claim.

On the day chief minister Abdul Malik took oath, five bullet-riddled bodies were found in the province.

The discoveries were seen by many as a message that security forces were intent on continuing what human rights groups have dubbed a systematic campaign of "kill-and-dump".

Jinnah stayed in the Quaid Azam Residency as he tried to recover from a lung disease in 1948, a year after his successful campaign to separate Pakistan from India. He died in Karachi soon after. The building is a national heritage site.-Reuters