Humayun Hamidzada, a spokesman for Hamid Karzai, the president, said: "As a principle, we shouldn't encourage kidnapping by accepting their demands."
In March, Karzai authorised the freeing of five Taliban fighters for the release of an Italian reporter, but called the exchange a one-time deal.
Tom Casey, a spokesman for the US state department, said there was regular contact between US and South Korean officials on the standoff, but would not comment on specifics.
Afghan judges killed
In a separate development, the bodies of four Afghan judges kidnapped nearly two weeks ago have been found in the same province where Taliban fighters are holding the South Koreans.
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| The bound feet of two of the judges [AFP] |
A photographer for Agence France-Presse, the French news agency, said he had seen the bodies of the four men.
He said one had been shot in the head, the others in the body, and that their feet had been bound.
Police said the bodies had been dumped in a village in the southern Ghazni province.
The Taliban had claimed to have kidnapped the four but did not immediately confirm it was responsible for their murder.
Mohammad Zaman, a deputy police chief, said the men were judges from the neighbouring province of Paktika.
He said: "They were killed and their bodies were found in Dehyak district last night."