TRIPOLI (Reuters) - Air strikes killed at least 15 people in the besieged eastern Libyan city of Derna late on Monday, a medical source and a resident said, in an operation condemned by the United Nations due to the high civilian death toll.

The U.N. Support Mission in Libya said at least 12 children and women were among the dead in Derna, a city about 265 km (165 miles) west of the Egyptian border controlled by a coalition of Islamist militants and rebel veterans known as the Derna Mujahideen Shura Council (DMSC).

The medical source said all the victims appeared to be civilians.

The coastal city has long been under siege by the eastern-based Libyan National Army (LNA), which has occasionally carried out air strikes against it, as has Egypt, which backs the LNA. Both denied carrying out Monday’s strikes.

The air strikes, lasting around an hour, hit the Dahr al-Hamar district in the south of Derna and Al Fatayeh, a hilly area about 20 km (12 miles) from the city, a resident said.

An Egyptian military source said Egypt was not responsible for the air strike, while one Egyptian TV station said Libyan planes had conducted it. The LNA denied this and said there had been a “terrorist attack” in the area.

Egypt’s foreign ministry issued a statement condemning the air strikes, which it said had killed innocent civilians.

The Tripoli-based U.N.-backed government, which opposes the LNA-backed by a rival administration in the east, denounced the air strikes and announced three days of mourning.

Islamic State established a foothold in Derna in late 2014, but was driven out by the DMSC the following year.