Sri Lanka President Gotabaya Rajapaksa has acknowledged for the first time that more than 23,500 people missing for a decade since the end of the country's protracted Tamil war are dead.

Rajapaksa, who played a key role in the military campaign that crushed the Tamil separatist rebels, told a UN envoy that steps would be taken to finally provide death certificates for those reported missing, his office said.

"President Rajapaksa outlined his plans to address the issue of missing persons," said a statement on the president's meeting with UN resident coordinator Hanaa Singer. "He explained that these missing persons are actually dead."

Some 5,000 security forces are among the 23,500 people never accounted for. The statement said most of the missing civilians had been conscripted by the Liberation Tigers of Tamil Eelam (LTTE) which was crushed in a major offensive that ended in May 2009. "The families of the missing attest to it. However, they do not know what has become of them and so claim them to be missing," the president said.