By Tom Miles

GENEVA (Reuters) - The United Nations has restarted air drops of food to support 93,500 Syrians besieged by Islamic State in the city of Deir al-Zor, the U.N. World Food Program said on Tuesday.

Air drops were halted on Jan. 15 after Islamic State militants cut the besieged zone in two and overran the drop zone that had been used for 177 air drops since April.

WFP spokeswoman Bettina Luescher said a new drop zone was now in use and air drops resumed on Jan. 29, with two drops at the new site so far. A total of 3,340 tonnes of food and other humanitarian aid have been dropped since last April.

Humanitarian experts say air drops are a last resort because they are complicated and deliver a fraction of the volume carried by a convoy of trucks.

Moreover, the air drops to Deir al-Zor are conducted at high altitude because of the threat of attack from the ground. A first attempt last year failed, with all the pallets blown off course or smashing into the ground after the parachutes failed.

Despite the interruption to WFP air drops, Syrian government helicopters managed to bring in some food and medicine, mainly for soldiers, as well as evacuating wounded troops, the U.N. humanitarian office said in a report on Saturday.

A private Syrian company also airlifted 24 tonnes of potatoes and onions into the city for sale at comparatively expensive prices, the U.N. said.

The U.N. air drops cost an average of $10,000 per tonne, a total cost of $33 million so far.

WFP food stocks in the city are almost depleted, and Red Crescent volunteers on the ground have been trying to put together food rations from pallets that were damaged in earlier air drops, the U.N. humanitarian report said.

This month's Islamic State offensive cut off two eastern neighborhoods containing 6,000 people, who now cannot access the hospital or get food or fuel. They drink untreated water, drawn from the river at night to avoid Islamic State attacks from the opposite bank.

The bakery in the east has also stopped working because of a lack of flour and fuel, although Russian helicopters brought in two days of bread supplies two weeks ago, the U.N. said.

The heavy fighting with Syrian government troops left the bodies of 40 child soldiers on the battlefield, the U.N. said, citing unconfirmed reports from local sources.

Medical sources reported 19 civilian deaths from shelling between Jan. 15 and Jan. 23, the U.N. report said.

(Reporting by Tom Miles; Editing by Dominic Evans)