A further calamity could take place very soon, he noted, and could have "a devastating impact" on neighbouring countries as well as on the situation in other parts of Sudan.

"What we are witnessing on the ground is a very serious degeneration of the situation," a UNHCR statement issued on Tuesday quoted Guterres as saying in London. "It is extremely nasty, with ugly events."

Since mid-September, the security situation has markedly deteriorated, with ambushes, hostage taking and attacks on villages increasing across Darfur. On 28 September, an attack on Aro Sharow camp for internally displaced people (IDPs) in West Darfur left 34 displaced people and local villagers dead.

Aid workers increasingly were the focus of attacks, Guterres noted. Humanitarian agencies said this was seriously hampering their capacity to operate on the ground.

On Sunday, IDPs in Kalma camp took more than 30 aid workers hostage in Darfur's largest camp on the outskirts of Nyala, the capital of South Darfur State, demanding the release of one of their leaders arrested by the Sudanese police, aid workers told IRIN on Tuesday.

Most humanitarian workers were released on Sunday and Monday. The last group of five aid workers, who were employed by the Sudanese water and environmental sanitation agency, were freed on Tuesday evening following protracted negotiations to secure their release.

As a result of a number of recent security incidents, the UN decided on 11 October to relocate non-essential staff out of parts of West Darfur as a precautionary measure.

Guterres, who visited Darfur two months ago, said at that time he was optimistic that, though not easy, peace would be possible in Darfur, and that if it came about it would have a beneficial impact on the rest of the country.

"You have three different crises at the moment," he said. "South Sudan, where peace was established based on the sharing of oil revenues; you have Darfur, and you have eastern Sudan, where the implications are also in relation to the neighbours and the problem between Eritrea and Ethiopia."

"Darfur ... in my opinion is the key for success or failure for Sudan as a whole," he said. "If there is success in Darfur, it will have a positive impact for coordinating a peace agreement in the south and for allowing peace to develop in the east."

The reverse, he warned, would probably produce the opposite result: "If it gets worse in Darfur, it will deteriorate, and even in the south the agreement will be weakened."

He expressed deep concern about the possible adverse effect on other countries in the region, especially Chad, which already hosts more than 200,000 Sudanese refugees from Darfur.

Much would depend on the extent to which the Sudanese government could guarantee security for displaced people and aid workers in its territory, Guterres noted.

He also urged the international community to refocus its efforts to reinvigorate the peace talks and give the 6,000-strong African Union force the ability to operate effectively.

The Darfur conflict erupted in February 2003 when the two main rebel groups, the SLM/A and the JEM took up arms to fight what they called the discrimination and oppression of the region by the Sudanese government. The government is accused of unleashing a local militia - known as the Janjawid - on civilians in an attempt to quash the rebellion.

Some 3.3 million people continue to be affected by the conflict, according to the United Nations, of whom 1.8 million are internally displaced and 200,000 have fled to neighbouring Chad.

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