South Korea's intelligence chief has refused to deny that his government paid a ransom to the Taleban to release 19 hostages last week.
The hostages were released after being held for six weeks
Kim Man-bok admitted to a parliamentary committee that there were undisclosed terms involved in the deal with the Afghan rebels.
He said the truth would be made public at some point.
South Korean officials had previously repeatedly denied reports that money was paid for the hostages' release.
"It's not proper for you, intelligence committee members, to ask whether a ransom was paid. I think it will serve better to leave it there for a while," Mr Kim said, according to South Korea's Yonhap news agency.
19 July: 23 South Korean Christian volunteer aid workers seized on a bus between Kabul and Kandahar
26 July: One male hostage shot dead - identified as Bae Hyung-kyu, 42, a church pastor and leader of the group
31 July: Another male hostage, Shim Sung-min, a 29-year-old former IT worker, found dead
10 August: South Korean officials and Taleban start talks
14 August: Two female hostages handed over to the International Committee of the Red Cross as a goodwill gesture - leaving 19
29 August: 12 more hostages released - leaving seven
30 August: All remaining hostages released
Sun Byung-ryul, a parliamentary committee secretary, told the Associated Press that when asked whether a ransom had been paid, Mr Kim replied: "I can't confirm that. Various things happened and they will be revealed as time passed."
Mr Kim travelled to Afghanistan to take part in negotiations with the Taleban.
As part of the deal, Seoul said it agreed to pull its troops out of Afghanistan as scheduled by the end of the year, and to stop missionaries travelling to Afghanistan.
But there have been persistent media reports alleging that a multi-million dollar ransom was paid.
A Taleban representative in Ghazni province, where the hostages were held, told the BBC the South Korean government paid $20m (£10m), but two other Taleban sources told the BBC no ransom was paid.
Afghan officials have said a sum slightly under $1m (£500,000) was handed over.
Two of the 23 Christian aid workers were murdered by their Taleban captors. Two others were then released because of their ill health, and the others were freed after six weeks in captivity.