An oil-rich region straddling northern and southern Sudan remains a potential trouble spot three years after the signing of a peace deal that ended decades of civil war, a U.N. envoy said on Tuesday.

The 2005 Comprehensive Peace Agreement was a historic north-south pact that ended two decades of civil war and promised southerners a referendum in 2011 on whether to split from Sudan and form their own country.

Tensions have worsened recently in the north-south border region due to the failure of Khartoum and the former rebel Sudan People's Liberation Movement, or SPLM, party that now leads the south to reach an agreement on the demarcation of the boundary of Abyei, the source of much of Sudan's energy reserves.

"The situation in the Abyei area has not changed materially ... and the area remains a potential flash point for the resumption of conflict," U.N. special envoy to Sudan, Ashraf Qazi, said in the written text of a speech he delivered at a closed-door session of the U.N. Security Council.

The status of Abyei was left unresolved in the 2005 peace deal.

There were clashes around Abyei in December and January between southern troops and Misseriya tribesmen. South Sudan's president, Salva Kiir, said the Misseriya were being supported by elements from the northern Sudanese army.

A report by U.S. human rights group Enough described Abyei in January as "Sudan's Kashmir" that could spark another civil war if left unresolved.

"The people of Abyei have been denied the dividends of peace since the signing of the CPA," Qazi said. "They have been deprived of ... basic service related to the provision of security, education, health and employment."

He said the issue of Abyei was the "biggest stumbling block between the two partners" - the SPLM and the National Congress Party, or NCP, of Sudanese President Omar Hassan al-Bashir.

The NCP and the SPLM were the two main partners in the power-sharing deal that came out of the 2005 peace deal.

Sudan's north-south war claimed 2 million lives and drove more than 4 million from their homes. But difficulties implementing the agreement to end the war and continued tensions have been overshadowed by international attention on a newer conflict in Sudan's western Darfur region.

DISPUTE OVER OIL REVENUES

An important dispute concerns the so-called Abyei Boundaries Commission's report on the demarcation of Abyei's border, he said. The NCP has rejected it, while the SPLM has accepted it.

That is a crucial issue, Qazi said, because demarcation of the border "will impact on ... the national census, elections, sharing of oil revenues and redeployment of forces."

The issue of Abyei's oil wealth is especially divisive.

But Qazi said a new ad hoc border commission was expected to begin mapping out suggested frontiers soon and should present its recommendations in the first quarter of 2008.

There are other problem areas.

"Little progress has been made on the issue of disarmament, demobilization and reintegration ... largely owing to the atmosphere of political mistrust which provides few incentives for the parties to downsize their armed forces," Qazi said.