A suspected female suicide bomber killed three Chinese teachers in Karachi on Tuesday, police and officials said, in the first major attack this year against nationals of long-time ally China working in Pakistan.

The three were among passengers on a minibus returning to Karachi university after a lunch break when the bomb exploded at the entrance to the university's Confucius Institute, killing the Chinese teachers and a Pakistani national, police and officials said.

A separatist group, the Baloch Liberation Army (BLA) based in southwestern Balochistan province bordering Afghanistan and Iran, claimed responsibility for the blast, adding in an email to Reuters the attack was carried out by a woman suicide bomber.

It shared in the email a photo of her clad in a long shawl sitting with two children. The photo couldn't be verified independently by police or other officials.

Karachi police chief Ghulam Nabi Memon said, "the reports we have got say they're Chinese." He added they were teachers at the Confucius Institute, a Chinese language and cultural centre.

"The information we've got is that the female bomber was most probably a student at the university," Memon told local Geo News TV.

A local guard and another Chinese citizen were also wounded in the minibus.

Local media showed CCTV footage of a woman dressed in black wearing a backpack standing close to the bus shortly before the bomb went off and sent up billowing clouds of fire and smoke. Police did not verify the footage. Local media showed the wrecked minibus dotted with shrapnel holes, and witnesses said the explosion was so big it rattled the windowpanes of other buildings on the sprawling campus.

The bombing was the first major attack against Chinese nationals in Pakistan since July last year when a suicide bomber blew up a passenger bus in northern Pakistan that killed 13 people, including nine Chinese working on a hydro-power plant. read more

Other attacks on Chinese working in Pakistan have taken place in Balochistan province, where separatist militants have waged an insurgency against authorities for decades.

Balochistan houses a deep-water port in Gawadar city which Beijing is developing under the China Pakistan Economic-Corridor (CPEC) project as part of President Xi Jinping's Belt and Road initiative to seek trade linkages with rest of the world.

The incident poses a major challenge to Pakistan's newly elected Prime Minister Shehbaz Sharif who took power this month. He condemned what he called a cowardly act of terrorism.

"I am deeply grieved on the loss of precious lives including of our Chinese friends in the heinous attack in Karachi today," Sharif said in a statement. He promised a speedy investigation. The Baloch separatist guerrillas, who say they are fighting for a greater share in regional resources of mines and minerals, usually attack gas projects, infrastructure and the security forces.

They also attack Chinese projects and workers despite Pakistan's assurances that it is doing everything it can to protect the projects. Islamabad blames arch-rival neighbouring India for backing the insurgents, a charge New Delhi denies.