Thailand was the largest violator of UNTAC. The Thai government at the time insisted that Cambodian imported timber must have a certificate of origin approved by the governmental authorities in Phnom Penh. These certificates cost US$35 for each cubic metre of timber from Khmer Rouge operating areas. This forced the Khmer Rouge to increase prices. They learned to speak Thai and sold timber illegally to Thai timber operators, earning them over US$10 million monthly. Global Witness, an international human rights and environmental non-governmental organization (NGO) based in London, recognized these timber guerillas when they identified mass Cambodian exports. They subsequently lobbied for an amendment to the US Foreign Operations Act. The act was passed. It stated that US assistance would no longer be given to any country cooperating militarily with the Khmer Rouge. Thailand closed its borders with Cambodia the next day.