Afghans warned of military action

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.

Brinkmanship continues

Afghan President Hamid Karzai has exchanged prisoners for hostages before, but the government has made it clear this is no longer the policy.

S Koreans react to killings

In pictures: S Korean shock

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.

Bodies discovered

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.

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.

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