A UN Security Council delegation has ended a visit to Sudan by urging the country to ensure a referendum on southern independence is held on time.
The timetable for January's vote was extremely tight but "doable", it said.
The referendum is the result of the 2005 Comprehensive Peace Agreement that ended the 21-year civil war between North and South, and UN diplomats fear any delay could cause renewed violence.
Supporters of the two sides clashed earlier on Saturday in Khartoum.
A crowd of several thousand northerners demonstrating in favour of unity turned on around 40 southerners who arrived at the rally. The police then joined in, beating southerners who fled the scene.
The scuffle was a rare public sign of the tensions threatening to split Sudan apart, says the BBC's James Copnall in Khartoum.
South Sudan's President, Salva Kiir, meanwhile asked the UN delegation to deploy peacekeepers along the border with the north ahead of the referendum.
Between eight and 10 areas along the border, including the Abyei oil fields, are still in dispute and analysts say a clash at any of them could spark off a military confrontation. Both sides have sent troops there.
Diplomats said Mr Kiir's request would be considered, but that no promises about a possible deployment had been given.
The visit by the delegation of 15 UN Security Council members focused on the civil war in the western Darfur region, and the delay in preparations for the referendum, which is scheduled for 9 January.
But the UK's Permanent Representative to the Security Council, Mark Lyall Grant, told reporters in Khartoum that they wanted to see a concerted push to resolve the many "key outstanding issues", such as funding and citizenship, before a vote could be held.
"With strong political will on both sides and the manifestation of that political will by implementation and strong support from the international community, we believe that that timetable is doable," he added.
On Darfur, the Security Council representatives called for the rebel movements not participating in negotiations to come back to peace talks without any pre-conditions. It also called on all sides to end hostilities.
On Thursday, government forces attacked rebel positions in the region.