Gunmen opened fire on passenger vehicles carrying Shiite Muslim civilians in restive northwest Pakistan on Thursday, killing at least 28 people, including six women, and wounding 30 others in one of the deadliest such attacks in recent years in the region, police said.

The attack happened in Kurram, a district in the northwestern Khyber Pakhtunkhwa province where sectarian clashes between majority Sunni Muslims and minority Shiites have killed dozens of people in recent months.

No one has claimed responsibility. The latest violence came a week after authorities reopened a key highway in the region after keeping it closed for weeks following deadly clashes.

Local police official Azmat Ali said several vehicles carrying passengers were traveling in a convoy from the city of Parachinar to Peshawar, the capital of Khyber Pakhtunkhwa, when gunmen opened fire.

He said the dead included six woman, and least 10 passengers were in a critical condition at a hospital.

Pakistan's President Asif Ali Zardari condemned the attack and offered his condolences to the families of the victims. He also ordered authorities to take action against those who orchestrated the attack.

Shiite Muslims make up about 15 percent of the 240 million population of Sunni-majority Pakistan, which has a history of sectarian animosity between the two communities.

Although they live together largely peacefully in the country, tensions have existed for decades in some areas, especially in parts of Kurram, where Shiites dominate.

Nearly 50 people from the two sides were also killed over the same dispute in July when clashes between Suni and Shiites erupted in Kurram

Pakistan is also currently carrying out intelligence-based operations in a separate conflict in northwest and southwestern Balochistan province, where militants and separatists often target police, troops and civilians, and the most violence has been blamed on Pakistani Taliban and an outlawed Baloch Liberation Army or BLA group.