Peacekeepers in Sudan say they are "gravely concerned" by a buildup of both rebel and government forces in North Darfur.
The joint United Nations-African Union peacekeeping force says the military escalation may signal a new cycle of fighting in the area.
The buildup is said be centered in the areas of Sortony and Kabkabiya. The U.N.-AU mission reports a "sizable and unusual increase" in military activities by the Sudanese government and the Abdul Wahid Faction of the rebel Sudan Liberation Army.
On Monday, the United States introduced a new strategy on Sudan, designed to end the conflict in Darfur and rising tensions between northern and southern Sudan.
Washington offered few details, but said the policy would combine incentives and disincentives, such as economic sanctions on the Khartoum government.
Sudan's government has been fighting rebels in Darfur since 2003. The United Nations says the fighting has killed up to 300,000 people and displaced more than 2.5 million others.
Sudan puts the death toll much lower, at about 10,000.
U.S. Secretary of State Hillary Clinton accused Khartoum of genocide Monday, a charge the Sudanese government denies.
Some information for this report was provided by AP and Reuters.