The United States warned Tuesday of an impending humanitarian catastrophe in Sudan's stricken western Darfur region unless Khartoum immediately opens the area to relief workers and disarms pro-government militias in accordance with a ceasefire.

In addition, senior US officials said Washington would hold back on easing sanctions on Khartoum – offered in return for a peace agreement with southern rebels in a separate conflict – until the crisis in Darfur is remedied.

"We have always told the government of Sudan: 'If there is a peace agreement (in the south), we will normalize relations with you,'" said Michael Ranneberger, the special US adviser for Sudan policy.

"Now we have said: 'Well, (even) if there is a peace agreement, we will not normalize relations with you until the Darfur thing is addressed.'" The Sudanese government must act immediately if it is serious in its stated commitment to resolve the critical situation in Darfur, where an armed conflict has killed more than 10 000 people and displaced one million since early last year, the officials said.

"Food is running out, sanitary conditions are terrible, disease is beginning to spread, the child mortality rates are rising at an alarming rate and we are facing a deadline," said Andrew Natsios, the chief of the US Agency for International Development (USAID).

With Darfur's mid-May-to-June rainy season fast approaching and threatening to cut off overland routes for relief convoys, hundreds of thousands of internally displaced people will be at risk without quick action, he told reporters at the State Department.

"If we do not have this resolved by the end of June, we are going to face a catastrophic situation by the fall," Natsios said.

He accused the Sudanese government of holding up a "massive relief effort" being prepared by the United States, the United Nations and international aid agencies by intentionally blocking access to Darfur and suggested Khartoum might be doing so in a bid to cover up widespread human rights abuses, including ethnic cleansing and systematic rape.

"Human rights organizations are telling us that the government is in the villages attempting to move mass graves, attempting to disguise some of the events that took place in the last six months," Natsios said.

Though he allowed that Washington had no independent confirmation of those reports, he said the United States had credible information that atrocities were still being committed by government-backed militias in Darfur despite the ceasefire reached earlier this month in neighboring Chad.

Khartoum has refused to grant visas to 28 special USAID disaster specialists who are ready to travel to Darfur to set up logistics for the delivery of 80 million tonnes of US food aid as well as medicine and temporary housing supplies that are either en route to Sudan or have arrived in the region's three main cities but await distribution, he said.

On Sunday, US Secretary of State Colin Powell called Sudanese Foreign Minister Mustafa Ismail to press Khartoum harder to allow those 28, as well as international relief workers, into the country and provide them with permits to travel to Darfur outside the main cities.

Natsios alleged that the government was withholding the visas for reasons "totally unrelated" to Darfur that have to do with accreditation of Sudanese diplomats in Washington. - Sapa-AFP