Islamabad, Pakistan (CNN) -- Gunmen ambushed a passenger van convoy Thursday in northwest Pakistan and executed 20 Shiite Muslims in an apparent sectarian attack, a police official told CNN.
"The gunmen forced the victims to step out of the vans, checked their identity cards, then shot them dead," said the police official, Sher Akbar.
The attack took place in the district of Mensehra, about 100 miles north of the capital, Islamabad.
The vans were traveling from Rawalpindi to Gilgit, a popular tourist city in the mountainous region of northern Pakistan with a large Shiite population.
In recent years, the northern regions of Pakistan have seen very little sectarian violence but this year, attacks targeting Shiites have increased.
In February, gunmen attacked a convoy of buses in the northern district of Kohistan, killing 18 Shiites.
The recent increase in sectarian violence began with the targeted killings of one or two people, said Muhammad Amir Rana, who heads an Islamabad-based think tank that monitors militant activity in Pakistan.
Lately, the violence has escalated into brazen and larger scale attacks like the one on Thursday.
More than 75% of Pakistan's population are Sunni; about 15% are Shiite.
The longstanding conflict between Sunnis and Shiites in Pakistan and other Muslim countries stems from a disagreement over who was the rightful successor of the Prophet Mohammed after his death.
Journalist Aamir Iqbal contributed to this report.