Five soldiers from UN-backed government forces on Friday were killed in airstrikes launched by Libya's eastern-based army in the city of Sirte, some 450 km east of the capital Tripoli.
"Five soldiers were killed by Haftar's air force as the troops were pursuing remnants of Islamic State terrorists in southern Sirte," said Taha Hadid, spokesman of Sirte's Protection and Security Force, the security service of the Sirte government.
The army, led by Khalifa Haftar, has been leading a military campaign in and around Tripoli since early April, attempting to take over the city.
Fighting has so far left thousands dead or injured, with more than 120,000 civilians displaced.
The United Nations Support Mission in Libya said it regrets the recent military escalation in the country, while stressing the importance of finding a political solution to the Libyan crisis.