Crucial reconstruction work following the devastating 2005 Pakistan earthquake is being disrupted following this week's mosque siege.
Plan has introduced curfews for women staff and restricted men from all but essential travel.
Attacks on NGO buildings and vehicles in the Siran Valley, North West Frontier Province, have already caused tens of thousands of pounds of damage.
Plan Pakistan communications manager, Natasha Kamal, says: "The danger is not from the communities that we work with, we have always been welcomed by them all.
"It is the fundamentalists working in the area that are the point of contention.
"The point is that all these extremists target NGO's as they have expatriate staff and their belief is that these organisations are here to promote western interests."
Plan is building 10 schools in the Siran Valley after the massive quake which killed 75,000 people across the region in October 2005.
But tension between Islamists and the secular government has intensified during the week-long siege of militants holed up in the Red Mosque, Islamabad.
A vicious 35-hour gun battle ended the stand-off on Wednesday during which 75 Islamists and 11 soldiers were killed.
Ms Kamal, aged 40, says: "The attitude of Plan communities is still the same, as they know through experience that we have always worked at supporting them.
"But there are so many other elements that are points of concern, like political parties, religious leaders and extremists.
"They don't want development because it makes people stronger and gives them the power to question them and their actions - so they harass organisations like ours.
"The feeling of unease and insecurity is there across the nation, not just with us, as these extremists are spread wide and far and they threaten every citizen of this country."
[ Any views expressed in this article are those of the writer and not of Reuters. ]
