BENGHAZI, Libya (Reuters) - A fighter jet from eastern Libyan commander Khalifa Haftar’s self-styled Libyan National Army was shot down in Dahr Al-Hamer district in Derna and its two crew detained, an air force spokesman said on Saturday.

Haftar’s LNA is one of the most powerful armed factions in Libya, where a U.N.-backed government in Tripoli is struggling to assert authority over an array of armed factions which have been competing for control since the 2011 fall of Muammar Gaddafi.

The coastal city of Derna - 348 km (216 miles) east of Benghazi - is under the control of a coalition of Islamist militants and ex-rebels called Majlis Shura Mujahideen Derna. The city has been besieged by Haftar’s LNA’s forces, who launch occasional airstrikes, for more than three years.

“A MiG-21 fighter was shot down by a missile after the aircraft carried out an airstrike targeting militants,” LNA spokesman for the Benina air base, Naser Hassi, told Reuters.

Derna, which has a history of militancy, is a target for LNA forces, backed by neighbouring Egypt and the United Arab Emirates, as part of Haftar’s self-declared war on Islamist militants.

In May, Egypt’s President Abdel Fattah al-Sisi launched airstrikes on militants in Derna in response to a deadly attack on Coptic Christians in Egypt. Critics said the strikes were meant to help Haftar rather than punish militants responsible for the Egypt attacks.