Sudanese officials during the weekend said their troops were dispatched to Abyei to drive forces from the southern government out of the area, al-Jazeera reports.
Abyei lies along the de facto border between Sudan and the future South Sudan, which gains formal independence in July. U.N. member states recently welcomed an agreement by both sides to recommit to a series of agreements on the departure of unsanctioned forces in Abyei in exchange for a joint police force.
The U.N. General Assembly in a statement condemned "the escalatory military operations being undertaken by the Sudanese armed forces, which have taken control of the area in and around Abyei town."
Abyei was left out of the January referendum that gave South Sudan the right to choose to become an independent state because of voter eligibility issues.
Susan Rice, the U.S. ambassador to the United Nations, said the Khartoum decision to seize Abyei was out of step with efforts at peace in the country.
"Our council takes the view … that this was an unfortunate and regrettable opportunity missed by the government of Sudan to share with the Security Council its perspective on what is clearly a very worrying crisis that has evolved," she said in a statement.