Walter Kaelin, a representative of the United Nations secretary-general, said civilians held in tightly-guarded camps should be granted freedom to ensure that the island complied with its international obligations.
"Immediate and substantial progress in restoring freedom of movement for the displaced is an imperative if Sri Lanka is to respect the rights of its citizens and comply with its commitments and obligations under international law," he said in a statement received here.
He criticised the slow screening of people in the camps for suspected Tamil Tiger rebels and called for unhindered humanitarian access to the camps by international and local aid workers.
Restoring freedom to the displaced "is becoming a matter of urgency, and I remain very concerned about the very slow pace of releases", Kaelin said, two days after wrapping up a visit to camps in the island's north.
Kaelin, UN representative on the human rights of internally displaced persons, said a clash over the we