Rebel armies on the move to central Sudan

UN regional co-ordinator for South Sudan David Gressley said the country was "on the brink".

The warning follows recent clashes in the oil-rich region of Abyei, which is claimed by both sides.

But Mr Gressley said the troop buildup in the area could unravel the 2005 peace agreement which ended two decades of civil war.

On Tuesday, the US envoy to Sudan warned that leaders from both sides were not interested in peace.

Leaders from both sides tried to downplay the fears of renewed conflict.

The warnings come as the UN Security Council is in Sudan, trying to ease tensions in Abyei, as well as the western region of Darfur.

Some 50,000 people - virtually the entire population - fled the heavy fighting in Abyei town last month.

The area contains oil wells which generate billions of dollars, but our correspondent says more than money is at stake - Abyei is coming to symbolise a new breakdown in trust between the mainly Arab north and South Sudan.

The two sides signed a peace deal in 2005 and the former southern rebels joined the national government.

Under this deal, the mainly Christian and animist south is due to hold a referendum in 2011 on whether to secede.