U.N. Secretary-General Ban Ki-moon expressed alarm over reports of a buildup of militia in the disputed Abyei border region as Sudan and South Sudan edge closer to all-out war.
South Sudan took control last week of the also-contested oil-producing Heglig region, in a neighbouring border state, prompting Sudan to vow to recapture the area by "all means".
In a sharp escalation of rhetoric, Sudan's parliament branded South Sudan an "enemy" on Monday and called for a swift recapture of Heglig.
The oilfield area is vital to Sudan's economy, producing about half the 115,000 barrel-a-day output that remained in its control when South Sudan seceded in July.
The U.N. statement did not give details of the reported buildup around Abyei and did not say where the reports were from but called it a violation of a June agreement in which both sides said they would withdraw their forces from the region.
"The Secretary-General is alarmed over reports received this weekend of the buildup of armed militia in the Abyei area," Ban's spokesman said in the statement.
"The Secretary-General calls on the Government of Sudan to ensure the full and immediate withdrawal of these elements from the area."
Abyei, which is prized for its fertile grazing land and produces some oil, was a major battleground during Sudan's civil war and is symbolically potent for both sides. Both countries lay claim to it.
Khartoum seized Abyei in May last year after a southern attack on an army convoy, triggering an exodus of tens of thousands of civilians. The Security Council authorised the deployment of 3,800 U.N. peacekeepers in Abyei in June.
Clashes in the last week have dampened hopes Sudan and South Sudan would soon reach a deal on disputed issues such as demarcation of their 1,800-km (1,200-mile) border, division of debt and the status of citizens in each other's territory.
South Sudan insists Heglig is rightfully part of the South and says it will not withdraw its troops unless the United Nations deploys a neutral force to monitor a ceasefire.
Some 2 million people died in Sudan's civil war, waged for all but a few years between 1955 and 2005 over conflicts of ideology, ethnicity, oil and religion.