A senior UN official told Reuters the threat of sanctions against Sudan could help secure a political breakthrough to end a crisis which has left 2.5 million people displaced.
John Negroponte, the US deputy secretary of state, accused Sudan's government on Monday of a campaign of intimidation against aid workers.
He also said that time was running out for Sudan to accept a hybrid force in Darfur or face new sanctions.
Negroponte said: "The denial of visas, the harassment of aid workers and other measures have created the impression that the government of Sudan is engaged in a deliberate campaign of intimidation.
"Time is running out."
George Bush, the US president, told Omar Hassan al-Bashir, the Sudanese president, last week that he had one last chance to stop violence in Darfur or the US would impose sanctions and consider other punitive options.
Britain has also stepped up the threat of sanctions.
Jan Eliasson, the UN secretary general's special envoy for Sudan, said: "I would hope that the awareness of the sanctions would play a role.
"I hope we'll not have to reach that stage, but it's a reminder of realities, of which I hope the parties to the conflict are aware."
Aid agencies said attacks on their operations had increased in the past three weeks. In one incident, a humanitarian convoy was shot at and robbed while travelling outside the town, the groups said in a statement.
An aid agency security guard was badly beaten in another attack and is in a critical condition.
"We greatly regret any suspension, even temporarily, of assistance to people in need," Oxfam and Save the Children Spain said in a joint statement. "But such attacks on humanitarian workers are not acceptable and cannot be tolerated."
The statement said a small number of aid workers would remain to monitor the situation and maintain essential services.
Aid agencies working in the town and in surrounding rural areas help displaced Darfuris as well as refugees fleeing the violence in Chad and Central African Republic.