The statement was made after the eastern-based army attacked the International Airport of the city of Misurata, some 200 km east of the capital Tripoli.
"Following media reports of airstrikes against Misurata's civilian airport, UNSMIL warns that attacks against civilian infrastructures and facilities constitute blatant violation of International Humanitarian law," UNSMIL said in a statement later Sunday.
"The mission calls for an immediate halt to such senseless attacks, especially against the only remaining civilian airport in western Libya, available for use by millions of Libyans," the statement said.
On Saturday, the eastern-based army said it targeted the International Airport of Misurata and destroyed Turkish-made drone control system used by the rival UN-backed government's forces.
The UN-backed government's forces said the airstrike injured a worker and caused material damage to the airport.
Since early April, the eastern-based army has been leading a military campaign against the UN-backed government in and around Tripoli, attempting to take over the city and overthrow the UN-backed government.
The fighting has killed and injured thousands of people, and also displaced nearly 120,000 civilians.
Libya has been plagued by political division and instability, ever since the fall of the late leader Muammar Gaddafi's regime in 2011.