Islamic State group jihadists and members of the Fajr Libya Islamist coalition engaged in fierce clashes on Wednesday in Sirte east of the capital Tripoli, an official said.
Mohamed al-Shami, head of press at Fajr Libya, said the fighting with IS took place in an area some 15 kilometres (nine miles) south of Sirte, home town of Libya's late dictator Moamer Kadhafi.
Shami said it began when the jihadists attacked a position manned by members of Infantry Battalion 166. One coalition fighter and 23 jihadists were killed, he added.
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There has been sporadic fighting between the battalion and IS for more than two months.
Battalion 166 has been tasked by the General National Congress, rival administration to the internationally recognised government, with restoring security in Sirte, 450 kilometres east of Tripoli.
After Kadhafi was toppled and killed in 2011, chaos descended on Libya as battle-hardened former rebel groups armed with heavy weapons carved out their own fiefdoms.
When the Fajr Libya alliance took Tripoli last year, both the recognised government and parliament took refuge in the east near the border with Egypt.
IS jihadists exploited the chaos to move in and have been gaining ground, notably in areas around Sirte.