United States President Barack Obama has described as "unacceptable" the expulsion of foreign aid workers from the Sudanese region of Darfur last week.
Mr Obama was speaking at the White House after his first talks with United Nations Secretary-General Ban Ki-moon.
Sudan expelled 13 aid agencies after the International Criminal Court issued a warrant for the arrest of President Omar al-Bashir for alleged war crimes.
The expelled groups deny Sudan's claim that they have a political agenda.
After his talks with Mr Ban, Mr Obama said he wanted to send a strong message with the UN to bring peace to Sudan.
"It is not acceptable to put that many people's lives at risk," he said. "We need to be able to get those humanitarian organisations back on the ground."
Earlier on Tuesday, the US embassy in Khartoum said it was allowing all non-essential embassy staff to leave the country.
The UN has said kicking out the humanitarian groups puts more than 1 million lives at risk.
About 2.7 million people are receiving aid in Darfur after being forced from their homes.
The UN estimates that 300,000 people have died, since black African rebels took up arms in 2003 against the Arab-dominated regime demanding a greater share of resources and power.
Mr Bashir on Sunday made a defiant visit to Darfur and warned all foreign workers to obey Sudan's laws or face expulsion.
The International Criminal Court issued a warrant for Mr Bashir on 4 March, accusing him of two counts of war crimes and five crimes against humanity in Darfur.
It is the court's first indictment against a serving president.
