The United Nations said on Tuesday that a quarter of a million people have been severely affected by the conflict in Sudan's southern border states, to which the government continues to deny the world body access.

"We consider there are over one million people who are quite badly affected by the fighting in Blue Nile and South Kordofan," Mark Cutts, the UN humanitarian agency (OCHA)'s head of office in Sudan, told reporters in Khartoum.

"We also consider that there are about a quarter of a million people who have been severely affected... Our main concern is for populations that are completely cut off from any relief supplies coming in from outside," he added.

Fighting in the border state of South Kordofan first erupted in early June, just weeks before the formal independence of the south, between the Sudanese army and fighters aligned to the SPLA, the ex-southern rebels turned regular army of South Sudan.

The conflict spilled over three months later into nearby Blue Nile state, another peripheral area where Khartoum moved to assert its authority in the wake of southern secession.

Cutts listed severe disruptions to the farming cycle – which is driving the region's growing food insecurity – as well as interruptions to basic services such as hospitals, health centres and schools, as examples of the conflict's social impact.

The UN children's fund (UNICEF) said in a separate report that cultivation levels in some areas are only 23 per cent of those in previous years.

"Food shortages and worsening nutrition levels seem certain to have a negative impact on morbidities and mortalities among children," UNICEF said.

It added that more than 50 youngsters were reported killed or wounded by aerial bombardment or crossfire.

Despite concerns about the worsening humanitarian situation, the Sudanese government has barred international aid workers, including all UN agencies, from accessing the region, a problem highlighted on Tuesday by the acting UN humanitarian coordinator in Sudan.

"We are in no position to verify the actual needs on the ground or the fulfilment of those needs as we are simply not there," Peter de Clercq said.

"The only thing that we can say with some confidence is that... we are looking at a deteriorating humanitarian situation in the areas out of which" the refugees in neighbouring South Sudan and Ethiopia have fled, both in terms of food and health, he added.

A Sudanese government official at the same news conference insisted that humanitarian workers were barred from the conflict zone for their own safety.

"Still there is fighting, still there is warring, there is kidnapping," said Mohammed Fadallah, acting humanitarian affairs commissioner.

"The government is doing its best to provide humanitarian assistance."

De Clercq said the UN humanitarian chief Valerie Amos was due to visit Sudan on Monday, to discuss "once again" the issue of humanitarian access to the war-torn southern states.

More than 80,000 Sudanese have now fled across Sudan's southern border since the conflict began, according to OCHA, while 417,000 remain displaced within South Kordofan and Blue Nile, mostly in areas controlled by the SPLA-North.

Meanwhile the fighting continues unabated.

A rebel spokesman said on Monday that heavy clashes in the Talodi area of South Kordofan over the weekend had left at least 19 government troops dead.

The Sudanese army has also carried out cross-border attacks on areas it says are retreat positions of the northern rebels, who Khartoum has long accused the south of providing with weapons and other supplies. Juba denies the claims.

Last month, a Sudanese army aircraft bombed the Yida refugee camp in South Sudan's Unity state, which houses more than 20,000 South Kordofan refugees, causing international outrage.