"To the extent that Sudan continues to frustrate implementation of this agreement, the US and other members of the international community are going to have to think seriously about implementing additional measures to deal with the humanitarian crisis in Darfur," said Casey.
He declined to go into further detail but in the past US officials have suggested a range of sanctions, including financial measures to pressure the Khartoum government into co-operating.
British officials are pushing for a non-fly zone to be imposed over Darfur, a tactic which they and the US say would ease the humanitarian crisis there.
In his letter President Bashir said he wanted to restrict UN movements, limit overflights or attack helicopters and bar international police from government-controlled zones and other areas.
An un-named State department official said: "People were surprised by exactly how far of a pulling back this represents".
"Clearly, Darfur needs help from the rest of the world"
Jack, Houston, USA
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The US statement come after a UN human rights report said Sudan's government had taken part in international crimes in Darfur and "failed to protect the population" in the region.
The team said the situation in the area is "characterised by gross and systematic violations of human rights".
"The mission further concludes that the government of Sudan
has manifestly failed to protect the population of Darfur
from large-scale international crimes and has itself orchestrated and participated in these crimes," the report said.
The report was commissioned by the human rights council in an emergency session last December.
After attempting to enter Sudan for around three weeks in February, the council concluded that the Sudanese government had no intention of co-operating with the UN.
The team was led by Jody Williams, a Nobel peace prize laureate, who told Al Jazeera that the international community needed to intervene more strongly in Darfur and that "words without action are irrelevant."
"People do not deserve to die because of the actions of a few men in Khartoum," she said.
The report urged stronger UN Security Council intervention, sanctions and criminal prosecution.
The violence in Darfur has claimed more than 200,000 lives and forced around 2.5 million people from their homes as rebels fight pro-government militias.
Some, including the US government, have described the violence as genocide.
The report said important steps have been taken by the international community, including the AU and UN, but "these have been largely resisted and obstructed, and have proven inadequate and ineffective."
|Around 2.5 million people have been|
displaced by the conflict [EPA]
Humanitarian agencies say they face increasing difficulties in getting help to those in need, and progressively more aid workers themselves are targets of the violence.
The government in Khartoum rejects the use of the term genocide and says the numbers pertaining to the conflict are exaggerated.
It blames rebel groups which refused to sign up to a peace deal in 2006 for the worst of the abuses and accuses the Western of blowing the conflict out of proportion.
However, last month Sudan's government was linked directly to the atrocities in Darfur, by prosecutors at the International Criminal Court in the Hague who named a junior minister as a war crimes suspect who helped recruit, arm and bankroll janjaweed fighters.
Although the UN team was unable to enter Sudan, it held consultations with several aid agencies and was briefed by AU officials in Addis Ababa.
They spoke to members of rebel groups, including the Justice and Equality Movement and the National Redemption Front, while to get witness reports of the suffering of the civilian population, they spoke to refugees from Darfur in eastern Chad.
Williams told Al Jazeera that Khartoum was keen to promote the idea the report lacked credibility as it opposed the investigation from the start and that such a response was "very predictable."
Sudan itself has rejected the report; their justice minister, Mohammed Ali al-Mardi described it as "worthless" and contradicted the report by saying that the humanitarian situation in Darfur was stable.
The UN council is due to debate the mission's report on Friday.
Source: Al Jazeera and Agencies