James Bays, Al Jazeera's correspondent in Afghanistan, said: "They [the Taliban] have extended [the deadline] in the past, but they say they are serious this time."
He reported there was a build up of security forces in the area where the South Korean were taken captive.
"There are a lot of troops and police in the area near the main highway where the South Koreans were kidnapped."
Talks stalled over the Taliban demands for prisoners held by the government to be freed.
"The government policy is not to release prisoners but the Taliban are still asking for prisoners," said Mahmood Gailani, one of the negotiators.
The Taliban has demanded the withdrawal of South Korean troops from Afghanistan and the return of prisoners held by the Afghan authorities.
Yousuf said on Sunday that the group's demands remained the same.
He said: "We have the same previous demands; the first is accepting to withdraw the Korean forces from Afghanistan."
"The second demand is still pending as the Afghan government delegation has said that it does not have the authority to release Taliban prisoners."
But Afghanistan's interior ministry and the presidential palace have said a hostage swap is out of the question.
An Afghan team that was supposed to have held more talks with the Taliban on Saturday could not reach the group because of security concerns in Ghazni province, a provincial source said.
The team hoped to persuade the group to free the Christian volunteers without condition.
The Taliban has said that if anything happens to the South Korean hostages, "the Afghan government and the South Korean government will be responsible".
The South Koreans were seized on July 19, while they were travelling on the highway between Kabul and Kandahar in Ghazni province, about 140km south of Kabul.
Source: Al Jazeera and agencies