Relatives of the hostages want the US to take action

They visited the US embassy in Seoul on Wednesday, and were told their plea would be passed to Washington.

The Taleban have already killed two of the 23 hostages, and threatened to shoot more if their demand of a prisoner release is not met.

South Korean officials have reportedly been holding talks with the Taleban to try to secure the hostages' release.

Mounting pressure

The 23 Korean Christian aid workers - 18 of them women - were seized on 19 July as they were travelling 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.

The family members who visited the US embassy in Seoul said they were determined to do all they could to save their loved ones.

"We will hold on to any small hope to save them," said Ryu Haeng-sik, whose 35-year-old wife Kim Yoon-yong is one of the captives.

"We cannot say we're relieved, but there is no other way but to believe their words, that they're going to do their best," he said of the American officials.

The Seoul government has also appealed for "flexibility" in negotiations with the Taleban - which analysts say is effectively a request for the US to make an exception to its policy of refusing to acquiesce to the group's demands.

The presidential office issued a statement on Tuesday, admitting that Seoul was powerless to grant Taleban demands, "because it doesn't have any effective means to influence decisions of the Afghan government".

The international community, and the US in particular, is seen to have much greater leverage.

But Afghan President Hamid Karzai has already refused to swap prisoners for hostages, after he was criticised for releasing five Taleban members from jail in March in exchange for an Italian reporter.

'Don't kill, negotiate'

As the hostage saga continues, anti-US sentiment in South Korea is rising, with local people angry that Washington has not stepped in to help.

About 100 protesters held a rally outside the US embassy on Wednesday, carrying banners reading "Bush: Don't kill, negotiate" and shouting: "The US is responsible for the killing of hostages".

US President George Bush "has a responsibility to intervene and negotiate with the Taleban", activist Oh Hye-ran told the Associated Press.

Outrage over the killing has also sparked increasing calls for South Korea to pull out its 200 troops in Afghanistan, or even to send these troops to rescue the remaining captives.