"Yet the United States has failed to respond to this genocide with the urgency that is required," they argued.
"As the death toll in Darfur continues to mount, it is clear that nothing short of international intervention can protect the people of Darfur," they wrote, adding: "We call on you to assert US leadership to ensure that such an international intervention takes place as a matter of the greatest urgency."
An uprising by the rebels in early 2003 prompted the government to unleash Arab militias in a scorched-earth campaign in which some 300,000 people have died and 2.4 million more fled their homes, according to a British parliamentary report.
UN Secretary General Kofi Annan is due to travel to Darfur this week to witness relief efforts and rally support for the AU peacekeeping presence.
The AU has a peacekeeping force of 2,200 in Darfur, which could rise to more than 7,700 by September