Sri Lanka said on Sunday it would allow UN aid workers access to civilians who remain in camps after the defeat of the Tamil Tigers, but only once rebels hiding among the refugees had been weeded out.
UN Secretary General Ban Ki-moon, on a visit to one camp housing 200,000 Tamils, had called for his staff to be given "unhindered access" to those displaced in the decades-long war, which ended a week ago.
Ban, who toured the Menik Farm facility on Saturday, described the conditions as overcrowded and the civilians as "badly in need of food, water and sanitation."
The government responded to his appeal for aid agencies to be allowed in by saying that "as conditions improved, especially with regard to security, there would be no objections to such assistance."
President Mahinda Rajapakse's statement warned of "the likely presence of Tamil Tiger infiltrators among the large numbers who had come to the government areas."
The government describes the camps as "welfare villages" and says it wants to resettle all displaced civilians as soon as possible, but Tamil activists say they are "concentration camps" with inmates penned in behind barbed wire.
Rajapakse told Ban that he would address the underlying ethnic grievances that lay behind the war, in which 80,000 to 100,000 people died in years of fighting between government troops and Tamil separatists.