Turkey has ordered Oregon-based Mercy Corps, one of the largest humanitarian organizations delivering aid to Syria, to immediately shut down its Turkish operations, ending a program that provides regular assistance to hundreds of thousands of besieged Syrian civilians and refugees.

“Our hearts are broken by this turn of events, which comes after five years of cooperation with the government of Turkey and other partners,” spokeswoman Christine Bragale said in a statement early Wednesday morning Turkish time.

It was unclear whether other aid organizations have been similarly affected, and Turkish government officials could not immediately be reached for comment. Bragale said that no specific reason had been given and that “we continue to seek a dialogue with Turkish authorities” and “remain hopeful that the government of Turkey will allow us to return to serve those in critical need.”

Turkey has admitted more than 3 million refugees fleeing violence in Syria, and towns and cities along its southern border have been a hub for sending humanitarian supplies into the war-torn country.