NATO became actively involved when its jets shot down four Serb aircraft over central Bosnia on 28 February 1994 for violating the UN no-fly zone. On 12 March 1994, the United Nations Protection Force (UNPROFOR) made its first request for NATO air support, but close air support was not deployed, owing to a number of delays associated with the approval process. On 20 March an aid convoy with medical supplies and doctors reached Maglaj, a city of 100,000 people, which had been under siege since May 1993 and had been surviving off food supplies dropped by US aircraft. A second convoy on 23 March was hijacked and looted.