NAIROBI, Kenya (AP) — At least 25,000 people were displaced after fighting in the town of Kodok, according to a spokesman for the humanitarian group Doctors Without Borders, one day after the top United Nations official in South Sudan warned of a government offensive on the town.

Phillippe Carr told The Associated Press that the fighting has caused a reduction in humanitarian services and there is a shortage of water for the displaced. Civilians are fleeing to the nearby South Sudanese town of Aburoc, Carr said.

On Tuesday, there were reports of shelling in the town of Kodok and on Wednesday the town was feared deserted with military helicopters sighted nearby.

South Sudan's government troops were on an offensive in Kodok and the Upper Nile region, head of the U.N. mission in South Sudan David Shearer told members of the Security Council on Tuesday. He warned that 70,000 civilians could be displaced.

"Virtually no part of the country is immune from conflict," Shearer told the 15-member body at the U.N. He added there "has been no concerted effort by any party to adhere to a ceasefire."

The government offensive in Kodok is one of the most significant of this fighting season, causing disarray among rebel leader Johnson Olony and his Shilluk forces, he said. It is also expected to create a flow of civilians fleeing into neighboring Sudan.

On Wednesday, the chief monitor of the peace deal, former Botswanan President Festus Mogae, said South Sudan was experiencing "a crisis within a crisis."

"Men, women and children are suffering and dying of starvation because the leadership at various levels is failing to prevent it," Mogae said.

The International Committee of the Red Cross also evacuated its aid workers from Kodok, spokeswoman Alyona Synenko told AP.

A spokesman for South Sudan's military was not available for comment.

The attack on Kodok comes after a flurry of recent government offensives across South Sudan and questions the legitimacy of an August 2015 peace deal signed by President Salva Kiir.

Since the August peace deal was signed the opposition has fragmented and the U.N. has noted the presence of ethnic cleansing and declared famine in two counties. Around 1 million people are at risk of starvation in South Sudan and 1.9 million people have fled the country.