UNITED NATIONS, June 8 (Reuters) - Fighting in western Sudan's Darfur region threatens the fledgling peace process in the south, the site of Africa's longest-running civil war, U.N. Secretary-General Kofi Annan warned on Tuesday.

The Sudan government in Khartoum and the rebel Sudan People's Liberation Movement signed a collection of protocols in the Kenyan capital of Nairobi earlier this month as a prelude to reaching a comprehensive peace deal for the south, expected to be ready in August.

But Annan, who is laying the groundwork for a U.N. peacekeeping mission to enforce the deal, cautioned in a report to the U.N. Security Council that an end to the separate crisis in Darfur "will be fundamental" to the success of a future U.N. role in the vast, oil-rich northeast African nation.

"To conduct a consent-based monitoring and verification operation in one part of the country while there is ongoing conflict in another part would prove politically unsustainable outside the Sudan and internationally," he cautioned, urging the parties in Darfur to quickly reach a separate agreement.

The view that the two Sudanese conflicts are linked is also shared by the United States.

While some Security Council members are opposed to mentioning Darfur in a pending council resolution on the Sudan situation, "we are pushing back strongly that not mentioning Darfur would be unconscionable," State Department spokesman Adam Ereli said in Washington on Monday.

MIRED IN WAR

Annan urged the Security Council to quickly send an advance team to Sudan to assess peacekeeping needs, saying it would demonstrate the international community's commitment to help.

Sudan has been mired in civil war for all but 11 of the 48 years since its independence from Britain in 1956.

The war in the south between the government and the SPLM erupted in 1983, when the Khartoum government tried to impose Islamic Sharia law on the animist and Christian south.

That conflict has killed more than 2 million people, driven 4 million from their homes inside Sudan and another 600,000 across borders into neighboring countries, Annan said.

In Darfur, Arab militias are driving out black Africans in what U.N. Emergency Relief Coordinator Jan Egeland has referred to as a scorched- earth campaign of ethnic cleansing. Two rebel groups launched a revolt in February 2003.

The fighting in Darfur has driven more than a million people from their homes and forced another 130,000 into neighboring Chad.

Annan warned of the huge challenges to the planning and execution of a U.N. mission in Sudan posed by its sheer size -- comparable to all of Western Europe, the complexity of the peace agreement in the south, and the country's war-ravaged communications and transportation facilities.

Without a patient commitment by world governments, "there is a real possibility that south Sudan could slide back into conflict and chaos," he said.