Rebels in Sudan's war-torn South Kordofan and Blue Nile states, where hundreds of thousands of people have been affected by fighting, called Wednesday for aid to be sent in through South Sudan and Ethiopia.

The request—which the rebels said they will formally make next week—comes as an international aid plan falls behind schedule despite warnings about the humanitarian situation in the two states along Sudan's southern border.

"We will ask for the cross-border operation from South Sudan and Ethiopia," Arnu Ngutulu Lodi, spokesman for the rebel Sudan People's Liberation Movement-North (SPLM-N), told AFP.

He said the Sudanese government "is not respecting" a memorandum of understanding signed on 5 August.

A ruling party official in Khartoum, however, insisted the government is committed to providing humanitarian assistance to all those affected by the war.

Both the government and rebels signed similar memorandums with the African Union, Arab League and UN to allow for humanitarian access throughout South Kordofan and Blue Nile—including in rebel-held areas.

The signing came six months after the three organisations submitted their proposal to Khartoum.

The UN has expressed concern for months about a worsening humanitarian situation in the war zone, where Khartoum cited security concerns in tightly restricting the operations of foreign aid agencies.

Lodi accused the government of "delaying tactics" and said the rebels would officially request the cross-border operation at a meeting between the signatories next Tuesday.

He said the rebels are still "committed to the MOU" and to working with the three foreign organisations.

Ali Adam, a director general of Sudan's Humanitarian Aid Commission, confirmed on Monday that implementation of the memorandum was behind schedule. He said the required planning meetings had not yet occurred.

"Our duty and our responsibility is to provide aid to our people in South Kordofan and Blue Nile," Rabbie Abdelatti Ebaid, a senior official of the ruling National Congress Party, told AFP. "Each and every and all support will be provided by our government."

Under the memorandum reached between Khartoum, the UN, AU and Arab League, the "entire humanitarian operations" are to be under Sudanese government supervision.

A United Nations source said the UN "has to respect the sovereignty of nation states" and could not enter a country without its permission.

But the source, declining to be named, said the UN "does understand why some humanitarian organisations, seeing what is happening... may wish to pursue" a cross-border operation.

"We will continue to work with the government to facilitate humanitarian assistance into SPLM-North areas under the memorandum of understanding that was signed with the tripartite," the UN source told AFP.

Ethnic minority insurgents of the SPLM-N were allies of southern rebels during Sudan's 22-year civil war, which ended in a 2005 peace deal and South Sudan's independence in July last year.

Sudan accuses South Sudan of supporting the SPLM-N, a charge which analysts believe despite denials by the government in Juba.

"We are not acknowledging the so-called northern sector of SPLM," Ebaid said.

The UN says more than 650,000 people have been displaced or severely affected by the fighting in South Kordofan and Blue Nile, where related conflicts began in June and September last year.

Government figures showed that more than 8,700 were uprooted in the latest fighting which continued this week around Al Moreib village in northeastern South Kordofan.

The rebels said on Friday that the government "campaign" against civilian areas aimed to stymie the international aid plan.

In total, more than 160,000 refugees have fled to South Sudan from South Kordofan and Blue Nile, the UN says.

In mid-August, the UN's World Food Programme began rare air drops of food to the refugees in South Sudan, calling the move "a sign of the increasing urgency" with much of the area inaccessible because of flooding.