Sri Lanka has made it clear that aid workers will not be allowed complete access to the thousands displaced by fighting until rebels hiding among the refugees have been weeded out.
United Nations secretary-general Ban Ki-moon had called for his staff to be given "unhindered access" to those who fled the war that ended a week ago.
He has left Sri Lanka after touring the island's embattled north and speaking with a few of the 300,000 ethnic Tamils displaced by the fighting.
Mr Ban described the conditions as overcrowded and the detained civilians as "badly in need of food, water and sanitation".
The government responded to his appeal by saying: "As conditions improved, especially with regard to security, there would be no objections to such assistance."
President Mahinda Rajapakse warned of "the likely presence of Tamil Tiger infiltrators among the large numbers who had come to the government areas".
During his visit, Mr Ban urged Rajapakse to probe alleged human rights violations committed during the defeat of the Tamil separatists, a joint statement on Sunday said.
The government responded warily to Ban's request, promising only to "take measures to address those grievances".
Between 80,000 to 100,000 people died in years of fighting between government troops and Tamil separatists, who were battling for an independent homeland on the Sinhalese-majority island, according to the UN.
The UN High Commissioner for Human Rights, Navi Pillay, has said both the military and the Tigers may have been guilty of war crimes, and campaign groups have condemned the army for indiscriminate shelling of civilians.
The Tigers have been accused of using civilians as a human shield.
Mr Rajapakse has robustly rejected all allegations of war crimes since his troops' victory, which came after he ignored repeated calls by the international community for a ceasefire.
Meanwhile, the Tamil Tigers have admitted for the first time their leader was killed by government forces.
Almost a week after the government said its troops had killed Velupillai Prabhakaran, 54, in an ambush, the Tigers' chief of international relations, Selvarasa Pathmanathan, acknowledged that the rebel leader was dead.