North and South Sudan Friday marked the fourth anniversary of the signing of a peace agreement that ended Africa's longest-running civil war.  But core issues remain unresolved and analysts say an external pressure not related to the accord is threatening to derail the agreement and plunge the country into a wider conflict.  

The government of South Sudan reportedly spent $21 million in the Upper Nile town of Malakal for this year's gala celebrations to mark the four-year anniversary of the signing of the Comprehensive Peace Agreement, better known by its acronym CPA.  

The Nairobi-based Sudan analyst for the International Crisis Group, Fouad Hikmat, says although much has been achieved since 2005, lingering mistrust between the governments in Khartoum and in Juba in South Sudan, among other problems, are preventing the two sides from resolving key issues vital to the full implementation of the peace deal.  

"The CPA managed to do a lot.  However, an agreement like this is not easy for the implementation," Hikmat said. "It takes time.  As we all know, there are key issues yet to be addressed."

Sudan's two-decade-long civil war erupted in the ethnically African south as a rebellion against the Arab-dominated Khartoum government and its control over the south's vast oil fields.  The war and related famines and diseases killed an estimated two million people in the south and displaced millions more.   

The CPA spelled out proposals for a fairer division of power and wealth, giving South Sudan autonomy and mandating the formation of a unity government with Khartoum.  South Sudan would also be able to hold a referendum in 2011 on independence from the North after national elections are held by this July.   

But a date has not yet been set for those elections, raising fears among southerners that a delay could push back the referendum on secession.

In May, a census was completed in South Sudan to prepare voter registration for the national elections.  Tensions are likely to rise if the census result shows fewer than 15 million southerners, a number that represents one-third of Sudan's total population.  According to the terms of the CPA, if it is shown that the South has less than a third of the population, Juba's share of the country's oil revenues may be reduced accordingly.

Another sticking point has been the failure of the North and South to agree on a common border. Some of the country's richest oil fields lie in areas claimed by both sides.  In May, northern and southern armies clashed in the town of Abyei in central Sudan for control over the oil-rich area.

International Crisis Group's Fouad Hikmat says he sees a more immediate problem for the CPA if Sudanese President Omar Hassan Bashir is arrested by the International Criminal Court over accusations of war crimes in a separate conflict in the western Darfur region.

If the Sudanese leader is indicted, Hikmat says the rebels in Darfur and elsewhere in Sudan are likely to intensify their fight against the Khartoum government.  He says more rebellions could irrevocably upset the balance of power that now exists between the North and South through the CPA.

"The issue of the indictment of the president is a new factor and I think the indictment could have a serious ramification toward stability in Sudan," he said. "If the international community wants to see Sudan moving toward stability in the coming period, it has to address the issue of the ICC."

A report released Friday by the London-based think tank Chatham House also urged the international community to step up efforts to ensure the success of the CPA.  Not doing so, the report warned, would allow the crisis in Darfur to be replicated across Sudan and lead to the country's fragmentation.