"Everybody saw what happened in Benghazi: disaster; institutions destroyed; houses demolished; mosques and universities burned by the criminal hands of Haftar's supporters," said the newly formed Mujahedeen Shura Council.
Forces loyal to former general Khalifa Haftar and Abdullah al-Thani, Libya's internationally recognised prime minister, are fighting to wrest full control of Tripoli and the eastern city of Benghazi from Islamist militants.
The jihadist Islamic State (IS) group that has seized chunks of Iraq and Syria is thought to have gained a foothold in Derna amid the chaos in Libya since the 2011 uprising that ousted dictator Moamer Kadhafi.
In its statement issued late on Friday, the Mujahedeen Shura Council called on everyone in Derna to join the new coalition.
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It also addressed Islamist fighters in Benghazi, saying: "We are with you in the war against the criminal Haftar and his soldiers".
Ahead of the announcement, the Islamists staged a military parade in Derna, with tanks and combatants carrying black flags.
Since the 2011 uprising, Libya has been sliding deeper into crisis, torn by rival governments, parliaments and powerful militias, despite UN efforts to broker talks on ending the violence.
Western countries have been increasingly worried that the political turmoil could provide fertile ground for Islamic extremists.
A top US military general said last week that the IS group has set up training camps in eastern Libya and that the American military is closely monitoring the situation.
Analysts say a number of factions in Derna have pledged loyalty to the IS group, but it remains unclear how much support they enjoy.