Bosnian air space, except for those expressly authorized by the UN Flight Coordination Center in Zagreb. The resolution also authorized UN member states to "take all necessary measures ... to ensure compliance" with the no-fly zone restrictions. In response to this resolution, NATO commenced Operation Deny Flight on 12 April 1993. Initially Deny Flight was intended only to enforce the no-fly zone; however several NATO members, including the United States, were eager to find ways to end the war and improve the situation of civilians, and hoped that military action could do so. The US had already taken unilateral actions to aid civilians caught in the conflict by dropping humanitarian supplies into Bosnia under Operation Provide Promise, and many US officials argued for the use of military force. These officials were eager to expand US air operations through Deny Flight, hoping that an aggressive no-fly zone and possible air strikes would end the conflict more quickly. NATO forces suffered its first loss on the second day of operations, when a French Mirage 2000 crashed in the Adriatic Sea due to mechanical failure. The pilot ejected safely. Enforcement of no-fly zone After its adoption, Operation Deny Flight was relatively successful in preventing fixed-wing aircraft from flying over restricted air space in Bosnia. During the monitoring phase of Operation Sky Monitor, unauthorized fixed-wing flights averaged twenty per month, but during Deny Flight, the average was three. During the conflict, there were only an estimated 32 fixed-wing military aircraft in Bosnia, all of them former Yugoslav National Army planes under the control of the Bosnian Serbs. Thus, NATO primarily needed to prevent incursions into Bosnian airspace from Croatia and Serbia. The first serious violation to the no-fly zone came on 28 February 1994, when six Serb J-21 Jastreb jets bombed a Bosnian factory. US Air Force F-16s shot down four of the six Serb jets over Banja Luka. This engagement was the first combat engagement of Operation Deny Flight, and its only significant air-to-air combat engagement. Perhaps more importantly, the Banja Luka incident was also the first combat engagement in the history of NATO. The Serbs acknowledged the loss of a fifth aircraft in the incident. While Deny Flight was relatively successful in stopping flights of fixed-wing aircraft, NATO forces found it very difficult to stop helicopter flights, which presented a more complicated challenge. All sides in the conflict used helicopters extensively for