The authorities in Afghanistan have dropped leaflets to the south-west of the capital, Kabul, warning people of a possible military operation there.
There are reports of a military build up in Ghazni province
The warning was issued in Ghazni province, where 23 South Koreans were taken by the Taleban two weeks ago.
The BBC correspondent in Kabul says there has been a military build up but reports of an operation being launched to free the hostages have been denied.
Two of the Christian aid workers were killed earlier this week.
The Taleban are threatening more deaths unless their demands are met for the release of eight government-held prisoners.
Afghan President Hamid Karzai has exchanged prisoners for hostages before, but the government has made it clear this is no longer the policy.
The BBC's Alastair Leithead in Kabul says there have been house searches, a leaflet drop warning locals of forthcoming operations and troops building up in Ghazni province.
However, he says that earlier reports - quoting a district chief - of a military operation having been launched to free the hostages appears to be false.
The South Korean government has been opposed to direct action from the start but the negotiations have so far failed to bring the hostage crisis to an end.
Our correspondent says the troop presence may well be a tactic to try to bring the crisis to a peaceful conclusion.
Deadlines have been set that have passed but the stand-off and brinkmanship continues.
The Korean Christian aid workers - 18 of them women - were seized on 19 July as they travelled on a bus down the Kabul to Kandahar highway.
The aid workers' leader, Pastor Bae Hyung-kyu, was the first to be shot dead by the militants. His death was reported over the weekend.
Shim Sung-min, 29, was the second South Korean hostage killed
On Tuesday, South Korea confirmed that a second hostage had been killed - 29-year-old Shim Sung-min, a former IT worker.
Meanwhile, the bodies of four Afghan judges who were kidnapped a day after the South Koreans, have been found in Ghazni province.
The bodies were discovered on Tuesday night in Dehyak district, an official told the BBC. The Taleban have claimed responsibility for the killings.
Are you in South Korea? What is your reaction to the killing of South Korean hostages? What do you think should the Korean government do? Send us your comments using the form below:
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.