Pakistani authorities have told the charity Medecins Sans Frontieres (MSF) to close its operations in a militant-hit tribal district, the group says.
In a statement, MSF said the authorities had refused it a permit to keep operating in Kurram district, on the Afghan border.
It said it was "saddened by the decision", for which no reason had been given.
MSF has been providing healthcare in Kurram since 2004.
Kurram is one of Pakistan's Federally Administered Tribal Areas (Fata), which are governed according to local laws and customs.
They lie along the north-west border with Afghanistan, where a number of militant groups are known to operate.
Kurram is the only majority Shia Muslim region in a predominantly Sunni country and its capital, Parachinar, has seen three major bombings this year.
'Out in a week'
In its statement, MSF said it had been told that its No Objection Certificate - required by NGOs to operate in certain areas - had been refused.
It said measures to close services had started and "should be completed within the week deadline".
The group has been operating out of two hospitals in Kurram, providing inpatient treatment for children and premature babies, as well as other services. MSF also operates in Bajaur, another of the tribal agencies.
A Fata administration official in Peshawar told the BBC the order to close MSF's operations in Kurram came from the Temporarily Displaced Persons secretariat, and said only the army would know why MSF had been asked to pack up.
Strategically important area? By M Ilyas Khan, Islamabad
Pakistan started to restrict the activities of all foreign missions, including NGOs, after the Raymond Davis affair of March 2011 and the killing of Osama Bin Laden two months later. The policy, which comes from the military, is aimed at controlling the access of foreign, especially Western, countries to local information.
Over the years, almost all foreign NGOs have been cleared from the tribal areas, while only a few are allowed to operate in Khyber Pukhtunkhwa, which borders the tribal areas, and insurgency-hit Balochistan. They mostly employ local staff since foreigners are discouraged in these areas.
It is hard to say why the army would want MSF out of Kurram, but the district's strategic value may be a factor. Since Pakistan cleared militant havens from Waziristan in 2014, Kurram, especially its northern and eastern parts, has emerged as the new haven for the Haqqani network and allied militant groups.
If the US starts putting more resources into the Afghan conflict, many believe the area may become important for pro-Pakistan militants to regroup and launch counterattacks in Afghanistan.