"The situation has been worsening and worsening in Darfur. In the east ... (it) is also very complicated and the conflict between Ethiopia and Eritrea is only widening it," said Antonio Guterres, the U.N. Nations High Commissioner for Refugees.
"Even in the south things are not as easy as they were," he said referring to south Sudan, site of a long war between rebels and the government that ended with a peace deal in January.
Guterres was speaking to reporters before a meeting with Denmark's Development Minister Ulla Tornaes.
In October, Guterres said the world only had weeks to help restore the peace process in Sudan' Darfur region, where rebels are fighting the government, and prevent the country's slide back into civil war.
A few months ago there was a clear will to reach an agreement on the conflict in western region of Darfur, while the situation in the east was also improving. Since then things have got worse, Guterrez said.
"Sudan is without any doubt, from the perspective of a refugee agency, the worst problem we are facing in the world today," he said, adding "The window of opportunity is narrowing but it is still there."
In Darfur tens of thousands of people have been killed and more than 2 million have fled their homes since rebels took up arms in 2003 against the government. The fighting in Darfur is separate from the war in the south.
Rebels in the east of Sudan have also fought government troops and tension has risen recently between Ethiopia and Eritrea, Sudan's eastern neighbours, who fought a 1998-2000 border war.