company during the Suez Crisis while American administration of the Panama Canal ended as a result of the Torrijos–Carter Treaties in which the United States voluntarily renounced its concession over the Panama Canal Zone. Overseas military bases Overseas military bases such as the American-administered Thule Air Base in Greenland and Guantanamo Bay in Cuba or the British-administered Akrotiri and Dhekelia on the island of Cyprus are a distinct category of concession ceded by a host state solely for military purposes. The jurisdictional authority ceded to the administering military force varies from base to base depending on the agreement concluded by the host and administering state. For example, Khmeimim Air Base in Syria is leased to the Russian government for a period of 49 years and the Russian government exercises extraterritorial jurisdiction over the air base and its personnel while the British government administers Akrotiri and Dhekelia (designated as "sovereign base areas" under the treaty establishing Cypriot independence) as an overseas territory. Sterile zones at ports of entry The sterile zone at a port of entry is the area where arriving international passengers have not formally entered the country by clearing arrival customs and immigration controls, and departing passengers have formally exited the country by clearing exit immigration control. Sterile zones are most commonly found in international airports, while they also exist at certain seaports and land crossings. Despite their usual exemption from local immigration and customs laws, international zones at ports of entry are fully under the jurisdiction of the country where they are located and local laws apply. Persons caught committing an unlawful act (e.g. possession of contraband such as illegal drugs) in the international zone are liable for prosecution. Within international airports, transit passengers can usually take connecting international flights in the international zone without clearing customs and immigration controls, and in most cases do not require a visa. Some countries, however, require transit passengers of certain nationalities to hold a direct airside transit visa even when they would not need to pass through border controls. In order to exempt passengers transiting between international flights from clearing border controls, most international airports outside North America feature a sterile zone which only authorized employees and processed passengers with a valid ticket are allowed to enter. Two major exceptions are the United States and Canada, where airports typically have no international transit zones. All passengers arriving on international flights are