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Army spokesman Brigadier Ahmed Khalifa al-Shami said rebels "today attacked one of our positions in the area of Mshayish, 38 kilometres (25 miles) west of Kadugli," the capital of South Kordofan. 

"We pushed them back and lost one of our men, while 10 others were injured," he told AFP.

Rebels carried out several attacks in recent days but government forces were respecting the ceasefire, he suggested.

Meanwhile, the rebel Sudan People's Liberation Movement-North (SPLM-N) said the army had attacked one of its positions.

The government was "launching today its summer campaign despite the announcement of a ceasefire by President Omar al-Bashir," said SPLM-N spokesman Arnu Lodi, without reporting any casualties.

An army statement referred to "heavy human and material casualties suffered by the rebels" without giving further details.

Ethnic minority rebels in the southern state, who say they are politically and economically marginalised, have been fighting the Arab-dominated government since 2011. 

Fighting there and in Darfur and Blue Nile states have left thousands dead and millions displaced, prompting Khartoum to announce a unilateral ceasefire in June - which was extended by six months in January.

Last week, the top UN aid official in the country said South Kordofan and Blue Nile were no-go areas and thousands of people were without access to humanitarian relief.

But Marta Ruedas said aid workers were now able enter the most war-torn parts of Darfur, where the UN says fighting has killed more than 300,000 people and displaced 2.5 million since 2003.

Aid is still blocked from Blue Nile and South Kordofan, she added.

Conflicts over access to resources have complicated the insurgency and created further instability in the region.

In East Darfur, tribal clashes killed more than 20 people and wounded dozens in the Abu Karinka zone on Monday, an official said.