By Akhtar Soomro and Asif Shahzad
KARACHI, Pakistan (Reuters) -A suspected female suicide bomber killed three Chinese teachers in Karachi on Tuesday, police and officials said, drawing strong condemnation from Beijing, 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 could not 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 Geo News TV.
A guard and another Chinese citizen were also wounded in the minibus.
China's Foreign Ministry strongly condemned the attack and "demanded" Pakistan punish the perpetrators, protect Chinese citizens and prevent such incidents from happening again.
"The blood of the Chinese people should not be shed in vain, and those behind this incident will surely pay the price," it said in a statement.
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 clouds of fire and smoke.
Police did not verify the footage.
Pakistani media also 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.
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 Gwadar 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 expand trade linkages.
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 neighbouring India for backing the insurgents, a charge New Delhi denies.
(Reporting and writing by Asif Shahzad in Islamabad; Reporting by Akhtar Soomro in Karachi; Additional reporting by Gibran Peshimam, Charlotte Greenfield and Saud Mahsud, and Beijing newsroom; Editing by Alexandra Hudson and Stephen Coates)