Secretary of State Colin Powell is in Sudan's troubled Darfur region, assessing the humanitarian needs of hundreds of thousands of people who have been displaced by a civil war. As the secretary left Khartoum for Darfur, U.N. Secretary-General Kofi Annan arrived in the Sudanese capital for talks with government officials. He is to visit Darfur on Thursday.

Secretary-General Annan did not speak to reporters as he arrived in Khartoum to begin to make his own assessment of what the United Nations calls the world's worst humanitarian crisis.

More than a million people have been driven from their homes in the western region of Sudan in what human rights workers call a campaign of ethnic cleansing by ethnic Arab Janjaweed militias, supported by the government.

Some of the displaced people are living in squalid conditions in refugee camps in Darfur, others have fled over the border into Chad. Until recently, the government allowed little access for humanitarian relief supplies into the region.

Human rights groups have been calling on the United Nations to exert diplomatic pressure on Sudanese officials to end the bloodshed. Sudanese authorities have promised to disband all armed groups in the area, but the pledge has been met with skepticism by human rights groups.

David Mozersky, with the Nairobi office of the International Crisis Group, says a U.N. peacekeeping force may be necessary to protect people in Sudan who have been displaced by the conflict, what he calls IDPs, or internally displaced persons.

"We're pushing for a Chapter 7 resolution in the Security Council to authorize an international force to go in and provide safe havens for IDP populations inside Darfur so that they do have security and international humanitarian efforts can access the people in need and so they can do so in a manner where they're not constantly being attacked, and looted and raped by the Janjaweed," he said.

That might be difficult to achieve. U.S. Secretary of State Colin Powell said late Tuesday it would be very problematic to find the necessary troops for an international force that could be effective in patrolling the region, which stretches thousands of kilometers along the Chad border.

Mr. Powell made his comments on his flight to Sudan late Tuesday for a one day visit that also includes meeting aid workers in Darfur. Mr. Powell and Mr. Annan are scheduled to meet in Khartoum later Wednesday.

Black African insurgents in Darfur launched a campaign against government forces 17 months ago, because of what they say was neglect of the region by the predominantly ethnic-Arab government.

Rights groups charge that the government has used the janjaweed militias to attack Darfur's civilian population in response.