KHARTOUM – Darfur gunmen ambushed United Nations and African Union peacekeepers, wounding four, officials said on Tuesday, in the first serious violence since an international arrest warrant for Sudan's president was issued.

The joint UNAMID peacekeeping force said the attack marked a worrying escalation in attacks against its troops.

Unknown armed men opened fire late on Monday on a UNAMID patrol returning to el-Geneina, the main town in west Darfur close to the border with Chad, said spokesman Noureddine Mezni.

The International Criminal Court (ICC) issued an arrest warrant for Sudanese President Omar Hassan al-Bashir earlier this month, accusing him of war crimes during nearly six years of fighting in Darfur.

Some analysts have said the warrant could spark more violence in the western region, where peacekeepers have been caught in the middle of a conflict involving rebels, government militias, bandits and rival tribes.

"There have been numerous car-jackings and attempts to steal in west Darfur," said Mezni.

"But this attack on our peacekeepers marks a worrying escalation. We are in Darfur to keep the peace and this is totally unacceptable."

The four soldiers, one reportedly in a serious condition, were flown to the peacekeepers' base in El Fasher, North Darfur for treatment, he added.