JUBA, Sudan - A confidential U.N. report warns that the invasion by northern Sudan's military of the contested region of Abyei straddling the country's north-south divide could amount to "ethnic cleansing" if the tens of thousands of residents who fled are not able to return.

The U.N. human-rights report - dated Sunday and marked "Not for Public Citation or Distribution" - said the Khartoum government in the north may have carried out a premeditated plan to invade Abyei when Sudanese Armed Forces, or SAF, troops moved in May 21.

"The SAF attack and occupation of Abyei and the resultant displacement of over 30,000 Ngok Dinkas from Abyei could lead to ethnic cleansing, if conditions for the return of the displaced Ngok Dinka residents are not created," according to the report, obtained by the Associated Press on Friday.

Expressing its "grave concern" about the violence, the U.N. Security Council later Friday condemned Khartoum for seizing control of Abyei, "and the resulting displacement of tens of thousands."

It also called on the Sudanese military to "ensure an immediate halt to all looting, burning, and illegal resettlement" in Abyei and asked both the north and the south to withdraw their military forces from the area.

The Ngok Dinka is a black tribe that associates itself with Sudan's south, which is set to become an independent nation. The tribe fled oil-rich Abyei when northern troops and ethnic Misseriya - ethnic Arab cattle herders aligned with the north - moved in and looted homes.

The U.N. report estimated that between 15 percent and 20 percent of the homes in Abyei were burned.