JUBA, South Sudan  — The U.N.’s top aid official in South Sudan watched helplessly as armed men in uniform stole a car from an aid group. Thugs took $50,000 of goods from Mercy Corps. Tons of food have been stolen, and $500,000 taken from a bank.

With renewed warfare reverberating throughout the world’s newest country, wholesale looting of vehicles, equipment and supplies belonging to aid groups is crippling humanitarians’ abilities to help. There is much work to do: Even before the fighting broke out last month, the people of South Sudan, one of the world’s poorest countries, were suffering from dismal health care and little education.

Now it’s much worse and they need even more help: Thousands of people have been killed and hundreds of thousands displaced by a month of fighting in South Sudan between factions of the military as well as ethnic militias.

Armed men on Thursday looted the Doctors Without Borders office in the city of Malakal, said Louisa Markering, the group’s emergency coordinator. Computers and phones were taken.

“It’s getting more and more difficult,” said Markering, who noted that this was the second mass theft against the group since the conflict broke out. “If they steal our assets it’s going to be more difficult to do our work. … It makes our life difficult, if not impossible, if our security cannot be guaranteed.”

The thefts have been particularly widespread in Bentiu, the capital of the oil-rich Unity State, which was in rebel hands for much of the last month before government troops retook the city.

Toby Lanzer, the U.N.’s top humanitarian official in the country, watched men in uniform steal a car from an unnamed international aid group last week.

“Our efforts to stop it did not succeed,” Lanzer wrote on Twitter, where he noted that it wasn’t worth risking lives to save a car. “Looting of aid agencies’ property in (hash)Bentiu will result in less aid reaching civilians who need the most help.”

Mercy Corps plans to return to Bentiu on Monday. The group had been delivering water and sanitation to refugees at the U.N. camp there. Then it lost $50,000 in office equipment and a motorbike.

“Now our assets are gone and it affects our work in the region,” said country director Mathieu Rouquett.

Doctors Without Borders’ compound in Bentiu was also hit, with virtually everything stolen, said Ines Hake, the group’s medical coordinator in Juba.

Hake urged all sides to respect the integrity of medical facilities and allow all ethnic groups to seek medical care.

Even as the risk of hunger increases, particularly among displaced people, looters have taken at least 3,000 metric tons of food from the World Food Program, roughly 10 percent of the group’s food in the country. That’s enough to feed 180,000 people for a month.

The International Committee of the Red Cross reported looting at its facilities in Bor and Bentiu.

South Sudan’s military spokesman, Col. Philip Aguer, blamed rebels loyal to former Vice President Riek Machar for the thefts in Bentiu.

“They blew up the ammunition. They looted. They robbed the bank. They stole nearly $500,000 dollars,” he said. After government forces retook the control of Bentiu last weekend, Machar said, “there was nothing left.”

But government troops are also allegedly involved in the looting.

U.N. Secretary-General Ban Ki-moon this week condemned “the commandeering of humanitarian vehicles and the theft of food stocks and other relief items by both government and anti-government forces.”

The U.N estimates at least 20 cars have been commandeered by rebels in Bor and Bentiu. The top U.N. official in the country, Hilde Johnsons, called on Machar to instruct his forces to return looted goods.

An Associated Press reporter who visited Bentiu last weekend after South Sudanese government troops retook the town, saw a mud-caked U.N. SUV being driven by some of the soldiers.

In the conflict, ethnically motivated attacks have been frequent over the last month, and officials in South Sudan say the U.N. has gathered evidence of gruesome assaults. Some victims have been bound and others beheaded, the officials said. Victims have also told AP that attackers have forced them to participate in sex acts with family members.

The U.N.’s assistant secretary-general for human rights, Ivan Simonoviae, is scheduled to give a news conference Friday. The two officials who detailed the violence insisted on anonymity because they weren’t authorized to release the information.

Human Rights Watch, in a report Thursday, said the worst ethnic-based attack took place in mid-December, when 200 to 300 men from the Nuer ethnic group were jammed into a room that gunmen fired into, killing nearly everyone.