A Rwandan peacekeeper has been killed and three others wounded in a gun battle after trying to mediate a tribal dispute in Sudan's Darfur region, the African Union-U.N. mission said on Saturday.

"As a result of the ensuing fighting, four Rwandan peacekeepers were injured and, unfortunately, one of them died from his wounds," UNAMID said in a statement.

At least 28 people have been killed in renewed fighting between two Sudanese ethnic groups, tribal sources said on Saturday.

Battles between the Hamar and Maaliya tribes began Thursday in the border region of West Kordofan and East Darfur states, tribal leaders from each side told Agence France Presse.

Hundreds of supporters of detained former Sudanese prime minister Sadiq al-Mahdi rallied on Friday despite a show of force by police and security agents.

The National Intelligence and Security Service (NISS) arrested Mahdi, chief of the opposition Umma Party, on May 17 after he reportedly accused a counter-insurgency unit of abuses against civilians.

Sudan has carried out "unconscionable" violence including aerial strikes and aggression against civilians in its war-torn regions, the U.S. special envoy to the country charged Thursday.

A senior official from the ruling party in Khartoum disputed the allegation and said the envoy, Donald Booth, was not qualified to make such comments.

The U.S. embassy urged Sudan on Thursday to respect freedom of expression, after state security agents arrested former prime minister Sadiq al-Mahdi for reportedly accusing a counter-insurgency unit of abuses.

In a statement, the embassy expressed "deep concern" over the Saturday detention of Umma Party chief Mahdi.

The rains have already come to Daldako, where Sudan's Rapid Support Forces are digging in.

Troops from the controversial unit are on alert for rebel retaliation, two days after they say they seized this strategic area about 20 kilometres (12 miles) northeast of South Kordofan's state capital Kadugli. 

Fighting between government and militia forces erupted in a city in Sudan's Darfur on Tuesday, killing four and ending an uneasy calm after the region's worst unrest in a decade, sources said.

"They are fighting inside El Fasher town -- heavy fighting," one resident told Agence France Presse by telephone, adding he could not identify the combatants, although some were in uniform.

President Barack Obama announced Monday that the United States would offer an extra $50 million to help tackle a growing refugee crisis spawned by fighting in South Sudan.

The money will be sent to international and non-governmental organizations and be used to pay expenses of the State Department's Bureau of Population, Refugees, and Migration in the region, Obama said in a memorandum.

Britain summoned the Sudanese charge d'affaires on Monday in protest at a death sentence handed to a heavily pregnant Christian woman for apostasy.

The political director of the Foreign Office, Simon Gass, pressed diplomat Bukhari Afandi to urge his government to do everything in its power to have the decision overturned.

Sudan's three-month suspension of aid work by the international Red Cross is having a "severe" impact as armed conflict worsens in the country, the ICRC said on Monday.

Sudanese authorities on February 1 suspended operations of the International Committee of the Red Cross (ICRC), saying the Geneva-based organisation had violated guidelines for working in the country.