KHARTOUM (Reuters) - Sudanese authorities have arrested suspects in an attack that killed four U.N. peacekeepers last week in the strife-torn western Darfur region, state media said on Tuesday.

UNAMID, the world's largest peacekeeping mission, was deployed by the United Nations and the African Union in the arid western territory after fierce fighting in 2003 which forced hundreds of thousands of people to flee their homes.

Violence in Darfur, where mostly non-Arab rebels took up arms against the government in Khartoum, has ebbed from its peak in 2003-4, but international efforts to broker peace have failed to end the conflict.

Last week, unidentified gunmen ambushed Nigerian peacekeepers near the western town of El Geneina, killing four soldiers and wounding eight, according to UNAMID.

"Suspects involved in the killing of (U.N.) soldiers have been detained," Sudanese government radio said in a text message sent to mobile phones. It gave no further details.

The U.S. State Department said it was "appalled" by the attack.

A total of 42 peacekeepers have been killed since UNAMID was set up, according to the force.

The International Criminal Court has issued arrest warrants for Sudan's President Omar Hassan al-Bashir and other officials to face charges of masterminding atrocities in the region where Sudanese troops and allied Arab militias have sought to crush the rebellion.

Estimates of the death toll vary widely.

Sudan's government signed a Qatar-sponsored peace deal with an umbrella organization of smaller rebel groups last year, but the major factions refused to join.

(Reporting by Khalid Abelaziz; Writing by Alexander Dziadosz and Ulf Laessing; Editing by Jon Hemming)