Sudanese Vice President Ali Osman Taha and southern rebel leader John Garang were Monday consulting at length with their delegations before resuming a fifth straight day of talks aimed at ending 20 years of civil war, officials said, according to Agence France-Presse (AFP).

"Washington is closely watching this peace process and I came along to see the developments in the talks," Jeff Millington, a former U.S. charge d'affaires in Khartoum, now representing his country on the Inter-Governmental Authority on Development (IGAD) which is mediating in the talks, told AFP.

He then went into talks with rebel leader Garang and IGAD's chief mediator, retired Kenyan General Lazaro Sumbeiyo.

Taha met late Sunday night with Garang, the leader of the southern-based rebel Sudan People's Liberation Army (SPLA), in the Kenyan Rift Valley town of Naivasha, 80 kilometers (50 miles) northwest of Nairobi, to try to resolve key issues that have stalled the peace process in Sudan, Africa's largest country.

A source close to the talks described the negotiations between Garang and Taha as "tough."

Diplomats in the region see Washington's interest in brokering a peace deal for Sudan - where a civil war has since 1983 pitted the south against Khartoum - as a desire to participate in developing Sudan's largely untapped oil reserves.

Sudan produces some 300,000 barrels per day of crude from oil fields in the centre of the country, near the frontline of fighting between Khartoum and the SPLA, but experts say it has the potential for much higher output.

Much of the country's oil wealth is believed to be in the south and other areas controlled by the SPLA.

Talks between Khartoum and the SPLA struck a breakthrough accord in July 2002 granting the south the right to self-determination after a six-year transition period and exempting southern region from Islamic laws practiced in Khartoum.

A subsequent round of talks focused on how to share power and resources during the six-year interim period of self-rule for southern Sudan.

Taha and Garang are trying to break a stalemate on how to share power and resources, particularly oil revenues, during a six-year interim period of self-rule for southern Sudan provided for in an accord signed in July 2002 and which will be implemented when a comprehensive peace agreement is signed.

Another stumbling block in the talks has been the government's opposition to a clause in a draft deal drawn up by mediators of the regional Intergovernmental Authority on Development (IGAD) providing for a separate army for the south, under Garang's leadership, during the period of southern self-rule.

Khartoum has argued that the clause paves the way for the south's immediate secession.