TUNIS (Reuters) - Forces in eastern Libya said they had arrested a rival senior military official in Derna on Friday and the United Nations said it was alarmed by an increasing risk to civilians in the city.

A photo released by forces loyal to eastern-based commander Khalifa Haftar showed the arrested officer, Yahiya al-Osta Omar, sitting handcuffed in civilian clothes. Local reports said he had surrendered to protect his family.

Al-Osta told Reuters earlier this week that residential areas in Derna were being hit by artillery fire and air strikes as Haftar’s Libyan National Army (LNA) stepped up its campaign to seize the coastal city.

Derna, 266 km (165 miles) from the border with Egypt, is the only city in eastern Libya to elude LNA control.

After blockading the city from mid-2016, the LNA launched a ground offensive to capture it from a coalition of local anti-Haftar combatants known as the Derna Protection Force (DPF) last month.

The LNA is now estimated to control about half of Derna, with the front line around the shuttered hospital. Residents fleeing the city said DPF fighters had planted mines in the Shiha district, one of the neighborhoods they have withdrawn to.

Access to medical care has been cut off and supplies of food, water and electricity badly disrupted, according to residents and humanitarian workers.

“We are deeply alarmed at the escalating risks to the population,” said Elizabeth Throssell, spokeswoman for the U.N. High Commissioner for Human Rights, in a statement.

“There have been increasing allegations that civilians have been arbitrarily detained, while others have been prevented from leaving the city.”

Haftar is the dominant figure among eastern factions vying for power in the conflict that developed in Libya after a NATO-backed uprising toppled Muammar Gaddafi in 2011.

He has opposed an internationally recognized government in Tripoli, but has received support from regional allies including Egypt and the United Arab Emirates.

The Derna campaign has overshadowed international efforts to chart a way out of Libya’s turmoil, including at a summit in Paris last week.

The LNA has branded the DPF “terrorists”, accusing them of harboring foreign fighters and having links to al-Qaeda.

Prior to his arrest, al-Osta said such claims were false and there were no foreign fighters or al-Qaeda militants in Derna. He pointed out that fighters from the Derna Mujahideen Shura Council, as the DPF was previously known, had driven Islamic State militants out of Derna in 2015.

“The situation is tragic,” al-Osta said on Tuesday. “There is methodical bombing of civilians. The city is calling for help.”