US President George Bush will impose sanctions on Sudan if a strengthened UN force is not allowed into Darfur "within a short period of time".
The African Union force is struggling to halt the violence
Mr Bush said he was giving UN Secretary General Ban Ki-moon one last chance to reach a diplomatic solution.
Sudan said earlier this week it would allow 3,000 UN troops into Darfur to support 7,000 African Union troops but has not agreed to a much larger force.
Separately, a UN report accused Sudan of violating an arms embargo in Darfur.
President Bush said he had decided to give UN Secretary General Ban Ki-moon more time to find a diplomatic solution with Sudanese President Omar al-Bashir but made clear his patience was limited.
UN DARFUR PLAN
Phase 1 - UN financial backing for AU mission
Phase 2 - UN sends logistical and military support
Phase 3 - UN takes joint command of hybrid force
"President Bashir should take the last chance by responding to the secretary general's efforts and to meet the just demands of the international community," Mr Bush said.
Speaking in Chad on Tuesday, US State Department official John Negroponte said Khartoum's offer did not go far enough and he insisted that a full scale UN deployment of between 17,000-20,000 troops is now important and urgent.
Sudan's apparent change of heart in allowing phase two of the three-phase UN plan to end the violence in Darfur comes after months of international pressure.
The four-year Darfur conflict between rebels and pro-government Arab militia has seen more than 200,000 deaths and at least 2.4 million displaced.
Meanwhile, UK Prime Minister Tony Blair's spokesman has said that the UK will call for international agreement on a new UN resolution on Darfur.
Mr Blair will say that the Sudanese president is not complying with international law and is "playing the international community".
In a separate development, the report by a panel of UN experts says that the government of Sudan is violating an arms embargo by transferring military equipment and weapons into Darfur.
What is more, the report says Khartoum is operating white aircraft in Darfur with UN markings, says the BBC's Laura Trevelyan, who has seen the report.
The authors say it is a deliberate attempt to conceal the aircraft's identity so that from a moderate distance they resemble those used by the UN or the African Union, our correspondent says.
The UN secretary general's spokeswoman said Mr Ban viewed this report with deep concern.
Sudan's envoy at the UN dismissed what he called the report's allegations as fabrications and said Khartoum wanted an urgent investigation.