A British aid agency has recently launched an appeal for humanitarian aid for the Darfur region of Sudan and east Chad.
According to Oxfam, the war-torn country needs £5m ($10m) to help displaced people in the region who continue to flee from violence.
US official John Negroponte, who visited Sudan, says it is lacking humanitarian aid and has warned of wider sanctions.
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.
"The denial of visas and harassment of aid workers has created the impression that the government of Sudan is engaged in a deliberate campaign of intimidation," US deputy secretary of state John Negroponte said at the end of his tour of Sudan.
Mr Negroponte warned Sudan of isolation if it fails to stop harassment of humanitarian workers and rejects the deployment of UN peacekeepers in the war-torn region.
About 10,000 people in Darfur are fleeing from their homes each month and aid agencies are facing unprecedented difficulties in reaching them, Oxfam reports.
"This is the greatest concentration of human suffering in the world and an outrage that affronts the world's moral values," Penny Lawrence, Oxfam's international director said after a tour of Darfur.
The international aid agency is currently providing clean water, health and sanitation services to more than 500,000 people in Darfur and eastern Chad.
"Nearly 1 million people are not getting any aid at all and in some areas the aid efforts is under threat due to increasing insecurity," an Oxfam statement said.
In eastern Chad, some 375,000 people have fled from the conflict between rebels and government forces and at present aid agencies can only provide three litres of water a day to the people.
An appeal by the United Nations for funds to support displaced people in Darfur and Chad has so far only realised $40m of the required $173m.
