A bomb attack in Pakistan's largest city Karachi killed at least 45 people, including women and children, and wounded 135 others, police said.

The blast on Sunday hit close to an area dominated by minority Shia Muslims, senior police official Ghulam Shabir Sheikh said, though he said that the target of the attack was not immediately clear.

"The bomb also badly damaged two residential buildings. Women and children were among the dead and injured," Fayaz Lughari, police chief of Sindh province, of which Karachi is the capital, said.

He said the initial blast was followed by a second explosion. According to reports, the impact of the bomb blast triggered a gas explosion in the same neighbourhood.

Al Jazeera's Imtiaz Tyab, reporting from Karachi, said: "What I'm seeing is a scene of carnage and destruction in Abbastown, where these two explosions took place in front of two apartment buildings."

"This is a fairly dense residential area, and between 30 to 40 apartments are reported to be damaged," our correspondent said. "We also understand is that many people are caught under the rubble."

"The fact that this is a large Shia neighbourhood is already raising alarms, after attacks on Shia communities in neighbouring Balochistan took place last month leaving over 200 dead."

Our correspondent described rescue efforts were still underway, with "many local residents using their bare hands to find anybody underneath the rubble who may have survived the blast."

As of yet no group has claimed responsibility, "but fingers are being pointed at Lashkar-e-Jhangvi, the same group who carried out the bombings in Quetta targeting Shia," he said.

Targeted attacks

Sunni armed groups have stepped up attacks in the past year against Shia Muslims who make up about 20 percent of Pakistan's population of 180 million people.

Two brazen attacks against a Shia Hazara community in southwestern city of Quetta killed nearly 200 people since January 10.

Pakistani armed group Lashkar-e-Jhangvi claimed responsibility for the bombings, which ripped through a billiard club and a market in areas populated by Hazaras, which are mostly Muslim Shias.

Pakistan's intelligence agencies helped nurture Sunni militant groups like Lashkar-e-Jhangvi in the 1980s and 1990s to counter a perceived threat from neighboring Iran, which is mostly Shia.

Pakistan banned Lashkar-e-Jhangvi in 2001, but the group continues to attack Shias.

According to Human Rights Watch, more than 400 Shiites were killed last year in targeted attacks across the country, the worst year on record for anti-Shia violence in Pakistan.

The human rights group said more than 125 were killed in Baluchistan province. Most of them belonged to the Hazara

community.

Human rights groups have accused the government of not doing enough to protect Shias.

After the January 10 bombing, the Hazara community held protests, which spread to other parts of the country.

The protesters refused to bury their dead for several days while demanding a military-led crackdown against the Lashkar-e-Jhanvi group.

Pakistan's president dismissed the provincial government and assigned a governor to run Balochistan province.

No operation was launched against the militant group until another bombing in February killed 89 people.

The government then ordered a police operation and has said some members of the group have been arrested.

One of the founders of the group, Malik Ishaq, was among those detained and officials said he could be questioned to determine if his group's is linked to the latest violence against Shias.