The United Nations has received alarming reports of malnutrition in two Sudanese border states where the army is fighting insurgents, a senior U.N. official said on Wednesday.

Fighting broke out in June between the Sudanese army and SPLM-North rebels in South Kordofan and spread in September to the state of Blue Nile. Both states border newly independent South Sudan.

The violence has already forced about 417,000 people to flee their homes, more than 80,000 of them to South Sudan, the United Nations estimates. Locals have faced air raids and sporadic ground fighting, according to rights groups and refugees.

"I received alarming reports with respect to malnutrition and the food situation, particular in areas that are controlled by SPLM-North," Valerie Amos, U.N. Under-Secretary General for Humanitarian Affairs, told reporters in Khartoum.

She urged Sudan to lift a ban on international U.N. staff traveling to both border states.

Since the outbreak of fighting U.N. agencies and aid groups have only been able to keep small teams of local staff on the ground and the government has stopped any aid workers visiting areas where there has been fighting.

"We need to ensure that the U.N. capacity, which is there to support government efforts, is made up of a mix of U.N. staff, national and international, to make sure we have the right skill set of support," Amos said after talks with Sudanese officials.

Social and Welfare Minister Amira Fadhil told journalists the ban was there to protect foreign workers and would stay in place.

"We fear for the security of foreigners. That's why we think the presence of a Sudanese organisation makes sense. But we want to grant access as soon as possible," she added.

South Sudan declared independence in July, under the terms of a 2005 peace deal that ended decades of civil war with the Khartoum government.

Both Blue Nile and South Kordofan contain groups who sided with the south in the civil war and say they continue to face persecution inside Sudan.

SPLM-N is one of a groups of rebel movements in underdeveloped border areas who say they are fighting to overthrow Sudan's President Omar Hassan al-Bashir and end what they see as the dominance of the Khartoum political elite.

Sudan and South Sudan, who still have to resolve a range of issues including the sharing of oil revenues, regularly trade accusations of supporting insurgencies on each other's territory.

Their armed forces clashed at Jau in a region claimed by both sides last month in a rare direct confrontation.