A Rezeigat tribal spokesmen said the tribe had been looking for new pastures for its cattle when fighting erupted on April 23 near the south Sudan boundary.

Southern Sudan accused the northern government of attacking and reported a new assault on April 25, saying its troops had been forced to retreat.

It is the worst violence reported since Sudan's historic polls on 11-15 April.

The delayed results of the presidential elections being held in both Sudan and Southern Sudan are now due to be announced on April 26, the country's national elections commission has said.

The first multi-party polls in 24 years - a key part of the peace process for the divided country - were marred by irregularities and alleged fraud.

Those results that have been announced, mainly from the north, suggest President Omar al-Bashir's party has a strong lead.

He is widely expected to be re-elected while the Sudan People's Liberation Movement (SPLM) are likely to hold on to power in semi-autonomous Southern Sudan.

Friday's clash is certain to raise tensions, particularly in the sensitive border area, the BBC's James Copnall reports from Khartoum.