In August 1981, in the first incident of the Gulf of Sidra, two Libyan jets fired on U.S. aircraft participating in a routine naval exercise over international waters of the Mediterranean Sea claimed by Libya. The U.S. planes returned fire and shot down the attacking Libyan aircraft. On 11 December 1981, the State Department invalidated U.S. passports for travel to Libya (a "de facto" travel ban) and, for purposes of safety, advised all U.S. citizens in Libya to leave. In March 1982, the U.S. government prohibited imports of Libyan crude oil into the United States and expanded the controls on U.S.-origin goods intended for export to Libya. Licenses were required for all transactions, except food and medicine. In March 1984, U.S. export controls were expanded to prohibit future exports to the Ra's Lanuf petrochemical complex. In April 1985, all Export-Import Bank financing was prohibited.