A World Vision office in Pakistan was attacked by gunmen on Wednesday, leaving at least five staff members dead.

The international humanitarian organization said the attack was unprovoked.

Initial reports say the militants detonated grenades and opened fire on the office, which is located in the Mansehra District of Pakistan, north of the capital, Islamabad. The office received no threatening letters prior to the attack.

"World Vision today is mourning the brutal and senseless deaths of five members [of] our staff," the Christian organization said in a statement.

Seven additional employees have been hospitalized and one staff member is missing.

"Those who kill humanitarian workers must be reminded that they are not only killing their own country's residents, but also people seeking to improve the lives of victims of poverty and injustice," the statement on Wednesday further reads.

The staff consisted of local Pakistanis who were serving in relief and development work.

Up to 15 gunmen arrived in pick-up vehicles and began firing on the staff, according to UK's The Times. World Vision administration officer Mohammad Sajid said the militants took their mobile phones, "dragged people one by one and shifted to an adjacent room and shot and killed them."

World Vision established offices in the South Asian country, where Christians make up less than three percent of the population, in 1992 and was focused primarily on relief interventions. In 2001, the organization expanded its operations as it collaborated with other aid groups in the North West Frontier Province and Punjab Province on emergency relief assistance and community development initiatives. After a massive earthquake struck the country in October 2005, World Vision expanded further and began operating in the Mansehra District.

All of World Vision's operations in the country have been suspended indefinitely.