The United States is calling on Sudan to put an immediate end to the violence in its western Darfur region. Secretary of State Colin Powell holds the government in Khartoum responsible for the Arab militias' persecution of the black population in Darfur. Experts are increasingly seeing the violence as ethnic cleansing which would constitute a reason for military intervention under the United Nations charter. Mr Powell is visiting Darfur next week and has called on Sudan to allow international aid workers into the region. More than one million people have been forced to flee their homes in Sudan's civil war.