(CBS/AP) PESHAWAR, Pakistan A Pakistani official says gunmen forced 20 Shiites off buses in northern Pakistan and killed them.
The police official said the incident happened Thursday in the Naran Valley.
The official spoke on condition of anonymity because he was worried about retribution.
The Deputy Inspector General of police in Gilgit, Ali Sher, said the victims were traveling from Rawalpindi, near the capital Islamabad, to Gilgit, a mostly Shiite area.
There have been several such sectarian attacks in the region in the past. Many Sunni extremists do not view Shiites as true Muslims.
CBS News' Farhan Bokhari reports the attack comes at a sensitive time for predominantly Muslim Pakistan. On Friday, the country's Muslims will celebrate Jumatul wida - the last Friday of the Islamic holy month of Ramadan, which is an occasion that typically sees large gatherings of friends and family.
A senior government official tells CBS News Thursday's attack could provoke Shiite protests on Friday, and increase sectarian tension across the country.
Though a minority in comparison to Pakistan's Sunni Muslim majority, Shiites comprise a significant portion of the nation's population - between one fifth and one third of the populace, depending on the estimate.
Thursday's attack also comes on the heels of a brazen siege by the Pakistani Taliban - one of the most active Sunni insurgent groups in the region - on one of the nation's most important air force bases.
A team of nine, which has suspected links to the country's nuclear program, before dawn on Thursday, killing a security official in a heavy battle that ended with the insurgents dead and parts of the base in flames, officials said.
The attack on the base in Kamra, located only about 25 miles northwest of the capital Islamabad, was a reminder of the threat posed by the Pakistani Taliban despite numerous military offensives against their sanctuaries along the Afghan border.
The large air base hosts a variety of fighter jets, including F-16s, and contains a factory that makes aircraft and other weapons systems. Some experts suspect the base could be linked to Pakistan's nuclear arsenal because of the weapons development there and the presence of jets that could be used to deploy the bombs. The army has denied the base has any links, but the nuclear program's secret nature makes independent evaluation difficult.
The safety of the country's nuclear weapons has been a major concern for the United States. Western experts say Pakistan has about 100 nuclear weapons and is in the midst of a rapid expansion of its arsenal.
"The great danger we've always feared is that, you know, if terrorism is not controlled in their country, that those nuclear weapons could fall into the wrong hands," U.S. Defense Secretary Leon Panetta told reporters at the Pentagon on Tuesday.
The militants, some of whom were wearing explosives strapped to their bodies, attacked the base at around 2 a.m. with automatic weapons and rocket-propelled grenades, according to the air force.
At least one of the rockets hit a hanger holding a number of aircraft said air force spokesman, Tariq Mahmood. The rocket pierced the hanger wall, and shrapnel from the explosion damaged one of the aircraft parked inside.
After the rocket barrage, the attackers scaled the wall surrounding the air base, said Mahmood.
Guards inside the base then opened fire on the militants, and an intense firefight ensued, he said. In the initial exchange of gunfire one Pakistani soldier was killed.
Security forces, backed by a team of elite commandos, fought the militants for two hours and were finally able to retake the base, the air force said.
Nine militants and one security personnel were killed in the fighting. The head of the base, Air Commodore Muhammad Azam, was wounded in the shoulder, said Mahmood.