A White House statement released Saturday evening condemned "the offensive operations being undertaken by the Sudanese Armed Forces" in and around the border town of Abyei, where Sudanese President Omer Hassan Al-Bashir sent in the military and decreed the dissolution of the regional government.
"The United States calls on the SAF to immediately cease all offensive operations in Abyei and withdraw its forces from Abyei," the statement said. "Failure to do so could set back the process of normalizing relations between Sudan and the United States and inhibit the international community's ability to move forward on issues critical to Sudan's future.
There are some 20,000 residents in Abyei, which is in a disputed area since elections earlier this year divided the country into a predominantly Muslim north and Christian/animist oil-rich south.
The Sudanese government said from Khartoum 22 of its soldiers were killed and many more were missing after fighting with rebels in the past two days, The Sudan Tribune said.
A southern military spokesman told the BBC the North sent at least 5,000 troops to the area and aircraft had dropped bombs.
South Sudan is scheduled to become independent in July. The country as a whole has been embroiled in ethnic civil war for decades and Abyei remains a political flashpoint.
A United Nations Security Council meeting in Khartoum scheduled for Monday was canceled, the BBC said.