A surging number of hungry refugees are fleeing fighting in Sudan where some are reduced to foraging in the wild, the UN said Monday, amid new allegations of Sudanese air strikes.

In South Kordofan state, where insurgents deny being backed by South Sudan, a Sudanese air raid killed a mother and two children, the rebel Sudan People's Liberation Movement-North (SPLM-N) said.

Air raids also continued over the weekend against South Sudanese frontline positions, the South Sudanese army said, despite an African Union order last week that the two nations cease border hostilities within 48 hours.

Sudan denied bombing in South Kordofan or South Sudan.

There has been "a notable increase in the number of new arrivals" who have crossed the border from South Kordofan into South Sudan's Unity state, the United Nations humanitarian agency (OCHA) said in its weekly bulletin.

The refugees are fleeing fighting between Sudanese troops and the SPLM-N, it said.

An average of 234 people crossed into the South every day in April, compared with 84 per day in February and March, the bulletin added.

The most serious border clashes yet between Sudan and South Sudan raged in April around the Heglig oil region, which is part of South Kordofan state.

South Sudan occupied the Heglig area for 10 days and Sudan carried out air strikes over the border in Unity state.

Elsewhere in South Kordofan, SPLM-N rebels besieged the town of Talodi into early April and, after a lull, fighting in the area intensified later in the month.

"Newly arrived refugees told UNHCR that food shortages, concerns that they may not be able to reach Yida with the rainy season approaching and intense fighting in their places of origin have prompted them to move to Yida," said the OCHA bulletin, covering the week to April 22.

Some reported "that they were relying on wild food" because fighting prevented them from planting and there was limited food for sale, it said.

Yida refugee camp is a key destination for people fleeing the fighting in South Kordofan, which began in June last year.

A mother and her two children were killed on Friday when a Sudanese plane bombed a village around Umm Durain, about 40 kilometres (25 miles) southeast of the state capital Kadugli, said SPLM-N spokesman Arnu Ngutulu Lodi.

Both sides said there had been renewed fighting around the strategic town of Talodi, southeast of Umm Durain.

Sudan has cited security concerns in controlling access for foreign relief agencies to South Kordofan and Blue Nile state, where a similar conflict began in September.

The UN and others have warned for months that aid agencies need access throughout the area -- including to rebel-held zones -- to properly assess people's needs and distribute assistance to prevent a worsening of the humanitarian situation.

The ethnic insurgents in South Kordofan fought alongside southern rebels during the civil war which ended in 2005, before the South's independence last July.

Sudanese cross-border air raids that continued after the end of the Heglig occupation drew swift international condemnation.

But Khartoum says the South's continued support for rebels inside Sudan undermines the north's stability.

South Sudan also accuses the north of backing rebels on its territory, an allegation the north denies.

"There has been fighting in Wedakona, Upper Nile state, with Khartoum supported militias", Southern army spokesman Philip Aguer told AFP, adding the combat continued early Monday.

He said that over the weekend Sudanese warplanes dropped four bombs near a forward position of the South's army in Unity state's Panakuach, about 20 kilometres from Heglig.

Sudan on Saturday expressed confidence in the African Union but rejected UN Security Council involvement in efforts to end weeks of border clashes that have raised fears of a wider war.

The Security Council has started talks on a resolution that could allow sanctions against Sudan and South Sudan if they do not end their clashes which broke out one month ago.

Russia, a veto-wielding member of the Security Council, signalled its agreement Monday with the US-backed resolution, after Foreign Minister Sergei Lavrov met his Sudanese counterpart Ali Karti in Moscow.

Also Monday, Sudan continued to hold four foreigners captured by the country's army along the tense southern border.

They are reportedly in good health, Norway's ambassador said, but diplomats still had not been given access to them.