The UN World Food Programme (WFP) said it was unable to get to northern parts of the country held by the Liberation Tigers of Tamil Eelam (LTTE) and warned the food situation there was becoming critical.
In some areas fuel was being sold at 400 percent the normal price and piped water was available only four hours a day, the group said.
It added that 204,000 people had left their homes since May 2004 across northeastern Sri Lanka, where rebels are battling government forces to set up a homeland for the country's Tamil minority.
"Without better access, WFP will not be able to continue feeding displaced persons living in areas outside of government control," said Jeff Taft-Dick, WFP country director, in a statement.
"In spite of high-level negotiations between the United Nations and the government of Sri Lanka the issue of humanitarian access to areas not under government control has not been resolved," the statement added.
He told reporters the situation was "extremely frustrating" wit