Gunmen attacked a bus carrying Shiite Muslims in Pakistan's insurgency-torn southwest on Tuesday, killing 13 people in the second major sectarian attack to hit the area in two weeks, police said.

Pakistani has faced mounting criticism from its minority Shiite community and human rights groups for failing to prevent thousands of sectarian murders, which further destabilise the fragile US ally in the war on Al-Qaeda.

Tuesday's drive-by shooting happened on the outskirts of Quetta, the capital of oil and gas-rich Baluchistan, which borders Iran and Afghanistan.

"Four gunmen riding two motorcycles opened fire on a bus in the outskirts of Quetta," local police official Hamid Shakeel told the media. "The death toll has risen to 13. Two of the injured, who were in critical condition, died in hospital. Now 12 Shiite Muslims and one Pashtun have been killed in the attack," he said after initially putting the death toll at 10.

The Shiites were members of Pakistan's minority Hazara ethnic group and had been travelling from the Hazara-dominated town of Hazarganji to Quetta.

Television footage showed the bus engulfed in flames, with blood and passengers' luggage seen scattered at the site sealed off by a paramilitary and police cordon. Police said they had been escorting other Shiites in the area, but insisted they had not been informed of the other bus.

Up to 400 furious Hazaras demonstrated outside the Bolan Medical Complex where the wounded were taken for treatment, condemning the government for inaction over sectarian groups, said police official Wahid Bakhsh.

Baluchistan has become an increasing flashpoint for sectarian violence between Pakistan's majority Sunni Muslims and minority Shiites, who account for around a fifth of the country's 167 million population.

On September 20, gunmen killed 26 Shiite pilgrims after ordering them off their bus in Mastung, a district 50 kilometres (30 miles) south of Quetta.

It was the deadliest attack on Shiites in Pakistan since September 4, 2010 when a suicide bomber killed at least 57 people at a rally in Quetta.

Gunmen then killed another three Shiites on the outskirts of the city who were going to collect relatives who died in the first incident.

‘Failure of Pak authorities’

London-based Amnesty International said the killings highlighted the failure of Pakistani authorities to address sectarian violence across the country.

"These are not random killings but demonstrate the deliberate targeting of the Shi'a (Shiites) by armed groups," said Amnesty's Asia-Pacific director Sam Zarifi.

"These attacks prove that without an urgent and comprehensive government response, no place is safe for the Shi'a," Zarifi added.

The rights group said it had recorded details of at least 15 attacks specifically targeting Shiites across Pakistan. Continued failure to address sectarian violence will only exacerbate the general breakdown in law and order in Pakistan," it said.

Pakistan's own independent rights watchdog said the killers had been emboldened by a persistent lack of action against sectarian militant groups, which have been implicated in thousands of deaths in past years.

Tuesday's attack ‘exposes once again the diminishing writ of the state’, warned the Human Rights Commission of Pakistan (HRCP).

Baluchistan is also rife with Islamist militancy and a regional insurgency waged by separatists who rose up in 2004 demanding political autonomy and a greater share of profits from the region's wealth of natural resources.