Witnesses and aid groups in Sudan say at least 40 people have been killed in tribal clashes in Darfur.

The witnesses said Sunday at least 21 people were wounded in the violence Saturday.

Tribal conflicts are becoming an increasing concern in Darfur, where people are already struggling with fighting between rebels and government forces along with militias backed by Khartoum.  The United States has called the fighting genocide against the people of Darfur.

More than 200,000 people have been killed in Darfur since 2003.

That was when a conflict over land and water resources turned violent after non-Arab rebels accused the Arab-dominated government of neglect.

The government has been accused of arming militias, called Janjaweed, to crush the rebels in a brutal campaign of rape and murder.

Some reports say tribal members blamed Saturday's attack on the Janjaweed.

Some information for this report was provided by AP and Reuters.