Jacob Townsend, China and Afghan Opiates: China and Afghan Opiates: Assessing the Risk (PDF), su Silk Road Paper, Institute for Security and Development Policy, giugno 2005, p. 36. URL consultato il 3 febbraio 2017.
«The only border crossing is the Wakhjir Pass at an altitude of 4,927m, which is closed for at least five months a year and is open irregularly for the remainder.»