Islamabad, Pakistan (CNN) -- In an unusual attack in a relatively peaceful region of northwest Pakistan, assailants ambushed four passenger buses, pulled out Shiite males and killed 18 of them Tuesday, police said.
Pakistani officials swiftly condemned the attack, with President Asif Ali Zardari saying the "culprits of such a heinous crime will not be spared."
The attack took place in the mountainous Kohistan district in the Khyber-Pakhtunkhwa province, senior police official Muhammad Ilyas said.
The attack was unusual in both its scope and its sectarian nature in a region that normally does not see much militant activity.
Police official Khurshid Khan said this was the first time an incident of this magnitude had taken place in the area.
Sunni-Shiite Muslim violence has been minimal in the past and this is the first time an organized terrorist attack targeting either group has taken place," he said.
A Pakistani Taliban spokesman, Ahmed Marwat, told CNN that his group claims responsibility for the attack.
The interior minister, Rehman Malik, immediately formed a team to investigate the incident and promised a report within three days, the Associated Press of Pakistan reported.
The buses were carrying passengers from Rawalpindi to the city of Gilgit in northwest Pakistan, Ilyas said. Gilgit is an area with a considerable population of Shiites.
Assailants stopped the buses early Tuesday morning and ordered the passengers out, he said.
They then singled out the male Shiite passengers, lined them up and shot them, Ilyas said.
Eight others were injured in the attack, but Sunni passengers were unharmed, he said.
Even though the area is not known for militant activity, Pakistan -- a majority Sunni Muslim nation -- has a long history of sectarian violence. Shiites make up roughly 20 percent of a population of about 170 million.
Muhammad Amir Rana -- the head of an Islamabad based think tank that monitors militant activity in the region -- says in recent months tensions have escalated between Sunni's and Shiites in the district of Kohistan with a number of targeted killings. He says the killings were usually single deaths that didn't make headlines. Rana says local leaders were trying to bring peace between the sects in the region.
"I think this is a result of the rising tension in the region in the past couple of months," he said. "We've seen targeted killings there but nothing of this magnitude. This can destroy the peace talks that were taking place in the area between different groups."
In a similar attack in October, gunmen boarded a passenger bus and opened fire, killing 14 in Balochistan province.
Last month, more than a dozen people were killed in a bomb attack at a Chehlum procession in Punjab province. Chehlum is a Shia religious holiday that marks the end of a 40-day mourning period following the death anniversary of Imam Hussain, the grandson of Prophet Muhammad.
In September 2011 gunmen killed more than 20 Shiites pilgrims after ordering them off their bus in a district in the southwestern province of Balochistan.
One of the deadliest attacks on Shiites was in September 2010, when a suicide bomber killed at least 57 people at a rally in Quetta.
Elsewhere in Pakistan Tuesday, a Chinese woman and her Pakistani translator were gunned down in the northwest city of Peshawar, police official Tahir Ayub told CNN.
Two men on a motorbike opened fire on Jiang Hua and Suleman Shams while they were walking on a busy road in Peshawar, said Ayub.
Police have acquired Hua's passport and are conducting an investigation into the killings said Ayub.
CNN's Shaan Khan and Reza Sayah and journalist Saboor Khattak contributed to this report