Heavy rains this week flooded nearly 2,000 makeshift shelters in the camps, where people displaced by war have been detained since the government in mid-May crushed the leadership of the Tamil Tiger rebels.
"Involuntary confinement is especially a source of concern given the recent rains and given the coming of the monsoon season," said Eric Schwartz, the US assistant secretary of state for population, refugees and migration.
"It makes it all the more important that release from confinement be an issue that friends of Sri Lanka continue to raise," he told reporters.
Sri Lanka says it needs time to weed out suspected rebel fighters from the camps to prevent a revival of the Tigers' four-decade struggle for an independent Tamil homeland, one of Asia's longest and bloodiest conflicts.
Schwartz, who visited a camp in Sri Lanka last month, said the conditions were "not great.
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"There is concern about communicable diseases, especially when you're in a temporary facility," he sa