By Jonathan Beale
BBC News, Washington
The US has said it expects Sudan to prove its commitment to allowing a strengthened UN peacekeeping force into Darfur within a matter of weeks.
The conflict in Darfur has displaced more than 2.4 million people
US Deputy Secretary of State John Negroponte said time was running out for Sudan to allow the international peace keeping force in.
Recent talks with Sudan's president were not encouraging, he said.
US President George W Bush has already outlined possible sanctions if Sudan's president does not keep his promise.
Mr Negroponte has accused Sudan's President Bashir of continuing to place obstacles in the way.
He also said that the Sudanese government had failed to disarm the Arab militia it has been supporting for more than four years.
The US has in effect now issued an ultimatum to Sudan to take steps to end the violence or face further unilateral and international sanctions.
However, speaking to reporters in Washington following his visit to Sudan, Mr Negroponte appeared sceptical as to whether President Bashir would comply.
"My sense is always that these agreements tend to be grudging and they tend to leave a lot of questions open as to whether they will follow through with implementation," he said.
The Bush administration has agreed to give the UN Secretary General a short period of time to persuade Sudan to co-operate before pressing ahead with further sanctions.
There is no specific deadline but US officials say they will only hold off for a matter of weeks.