Sudan has stepped up air attacks in South Kordofan, a state on the south Sudan border, causing "huge suffering" to the civilian population and endangering emergency aid, the United Nations says.
Heavy fighting between forces from the north, including both the Sudanese Armed Forces northern army - or SAF - and government-back forces, and fighters aligned to the Sudan People's Liberation Army (SPLA), the former southern rebel group - has raged across South Kordofan since June 5.
"We are extremely concerned about the bombing campaign, which is causing huge suffering to the civilian population and endangering humanitarian assistance," Kouider Zerrouk, a spokesman for the UN Mission in Sudan (UNMIS), told the AFP news agency.
"The intensive bombing by SAF in the past week is continuing in Kadugli and Kauda, where jet fighters dropped 11 bombs at 1030 this morning, apparently targeting an airfield."
Two bombs landed very close to the UNMIS compound in Kauda, which is situated just 150 metres from the airstrip.
Fears had been growing among civilians of intensified SAF air attacks on former rebel strongholds, where the indigenous Nuba peoples fought on the side of the SPLA during the devastating 1983-2005 civil war between north and south.
"We reiterate our call on the SAF, the SPLA and other armed groups who are involved in this conflict to allow immediate access to humanitarian agencies, stop military attacks agianst civilians and respect and protect them in accordance with international law," Zerrouk said.
UNMIS was unable to provide details of casualties from the latest violence in South Kordofan.
Late on Monday, a Sudan rights group said that more than 65 people had been killed in air raids carried out by Antonov bombers in different locations around South Kordofan over the past nine days.
The Sudan Democracy First Group (SDGP), in a six-page report, accused the northern army of pursuing a genocidal campaign in the state, targeting the Nuba peoples and supported by the Popular Defence Forces, a feared civil war militia that now forms part of the Sudanese army.
A northern army spokesman, Al-Sawarmi Khaled, denied that Khartoum's military actions were killing civilians, saying fighting was only between the army and rebels.
"There are not any victims from the civilian people," he said.
But speaking to Al Jazeera on Tuesday, Tawanda Hondora, of the Africa department at Amnesty International, the London-based human-rights monitor, said: "Civilians currently are fleeing to other areas out of South Kordofan.
"Some of them have fled to the UN compound. Unfortunately, we have information that they are actually not safe. The UN is unable to provide them with security."
Hondora said the Sudanese armed forces have been going to the UN camps and taking people from there.
"Some of them have been shot and killed simply because they look to be Nuba, and therefore perceived to be sympathetic to the SPLA," he said.
The fighting in South Kordofan has raised tensions at a sensitive moment for Sudan, with the south set to declare independence in less than a month. The split has been complicated by a number of unresolved issues, including where to draw the common border and how to divide oil revenues.
Southerners voted to secede in a January referendum which was promised in a 2005 peace deal that ended decades of north-south civil war that killed two million people.
It was announced on Monday that north and south Sudan had agreed "in principle" to demilitarise the disputed central region of Abyei and to allow an Ethiopian peacekeeping team to come in.
The agreement was reached after two days of talks between Omar al-Bashir and Salva Kiir, the respective leaders of Sudan's north and south, according to Barney Afako, a spokesman for the former South African president, Thabo Mbeki, who is helping lead the negotiations.
"The parties have agreed to the idea that the Abyei area should be demilitarised and they agreed in principal that the southern and the northern forces are to pull out of the area," Afako said.
The political status of Abyei and issues such as oil revenues are yet to be discussed, Afako said.
To assist with the demilitarisation, the Ethiopian government offered to send in troops to oversee security so that civilians who fled can return, he said.