Azerbaijani forces on May 15 destroyed an Armenian anti-aircraft system and killed its crew in the disputed Nagorno-Karabakh region, the Azerbaijani Defense Ministry said, APA and Trend reported. The Osa surface-to-air missile system had been deployed near the de facto line of control in the breakaway territory, which is officially recognized as part of Azerbaijan but is effectively under the control of ethnic Armenian separatists with backing from Armenia. The Armenian Defense Ministry vowed to retaliate. No official casualty count has been given, but Osa systems are generally operated with five-person crews. The systems are capable of monitoring air targets within a radius of around 45 kilometers (27 miles) and of destroying targets within a radius of nine kilometers. The Nagorno-Karabakh region has been a hotbed of contention between Azerbaijan and Armenia since they were Soviet states, and today, their differences over the breakaway territory are as intractable as ever.