Qari Yousuf Ahmadi, Taliban spokesman, on Saturday said: "The Taliban leadership council has set 12:00 [07:30 GMT] as deadline for the two German hostages. And if the deadline is not met, we will kill them."
South Korea on Saturday reiterated its plans to withdraw troops from Afghanistan by the end of the year, as scheduled.
While Seoul has no combat troops in Afghanistan, it have a military contingent of about 200 engineers and doctors deployed in the war-torn nation.
Song Min-soon, South Korea's foreign minister, said he had spoken by phone to his Afghan counterpart and that officials from both countries were working to secure the Koreans' release.
He said a team would leave for Afghanistan later on Saturday, and that the Afghan side had already set up a special task force to deal with the case.
In the largest-scale abduction of foreigners since the Taliban was ousted from power in 2001, the South Koreans were taken at gunpoint from a bus in Ghazni province's Qarabagh district on Thursday as they travelled on the main highway from capital Kabul to the southern city of Kandahar.
On Friday, Taliban said they were holding 18 South Koreans. A South Korean aid group said 23 were missing.
The Taliban on Saturday said they actually have 22 Koreans, 16 of whom are believed to be female, says James Bays, Al Jazeera's correspondent.
He said: "There is still a discrepancy, but the figure seems most likely to be 22 or 23, not 18."
The Taliban has seized a number of foreign nationals as part of its campaign to overthrow the Afghan government and drive out its Western backers.