A total of 15,780 people were reported missing to the ICRC until the end of last year.
In its annual report, the ICRC president Jakob Kellenberger said that "thousands remained unaccounted for, leaving relatives without definitive information about their fate".
The missing individuals include 1494 children and 754 women.
The report said although ICRC staff made visits to around 17,700 detainees, they were unable to see anyone in the 'rehabilitation' camps.
"Thousands of people continued to be held in relation to the former armed conflict at temporary and permanent places of detention around the country, and arrests continued.
"When the state of emergency was lifted, inmates held in connection with the former armed conflict were detained under the Prevention of Terrorism Act.
"The ICRC monitored the application of such detentionrelated laws and abandoned its fruitless efforts to obtain access to people held in rehabilitation centres."
The ICRC criticised the Sri Lankan government for issuing an order to force the organisation to pullt out from northern Sri Lanka, which has severly affected war displaced.
The ICRC said that it had to stop several projects including 'the provision of seed and tools to farmers to boost crop production; the provision of tackle to fishermen to help them resume livelihood activities; cash grants or vocational training to vulnerable families to invest in improving their future; and the rehabilitation and/or extension of municipal water systems'.
'Over 15,000 missing in Sri Lanka' : Report reveals - JDS (03 Jul 12)
