Taliban militants shot dead a South Korean hostage today after a deadline for their demands passed, a spokesman said.
The body of the hostage was recovered in central Afghanistan, a police officer who discovered the victim said.
According to the officer, the man had been repeatedly shot in the head, chest and stomach, and was discovered in the Mushaki area of Qarabagh district, in Ghazni province.
Qari Yousef Ahmadi, who claims to speak for the militants, told agencies that one of the 23 hostages had been killed at around 4pm local time (12.30pm BST).
Mr Yousef told Reuters that the militants would kill the remaining hostages unless demands were met by a new deadline.
"If the administration of Kabul is not ready to release our hostages, then by 1am the rest of the hostages will be killed," he told Reuters. "That time is the last deadline."
A police official said militants told him the hostage was sick and could not walk, and was therefore shot.
A number of other South Korean hostages were freed and taken to a US base in Ghazni, western officials said. The officials did not know how many had been freed.
A South Korean news agency, Yonhap, said eight Koreans had been freed, citing unnamed Korean officials.
An Afghan official involved in the negotiations earlier said a large sum of money would be paid to free eight of the hostages.
Mr Yousef said previously that patience with the authorities was "running out" after Kabul failed to meet their demands to release eight Taliban prisoners by yesterday.
South Korea's foreign minister, Song Min-soon, said the hostages were being held in different locations and a "safe release may not be necessarily matched by a quick release".
Ali Shah Ahmadzai, the provincial police chief, said he thought talks had been positive and the new threat was a surprise.
"I don't know why they've suddenly changed their mind," Mr Ahmadzai said. "My message to the Taliban is to use tolerance and be patient. This [killing of hostages] is against the Afghan culture."
The hostages, including 18 women, were kidnapped last Thursday while travelling on a bus through Ghazni province on the Kabul-Kandahar highway, Afghanistan's main thoroughfare.
South Korean negotiators have travelled to Ghazni province to take part in the negotiations. Three previous deadlines over the hostages' lives have passed with no consequences.
Mr Yousuf told Reuters: "We had assurance from the Koreans that Kabul will release Taliban prisoners in batches and we will reciprocate. "We gave them eight Taliban names and they should have been freed by 7pm yesterday, but nothing happened.
"The Koreans should put pressure on Kabul on this for there is the risk that at any moment, any time something can happen to the hostages. If by two o'clock today, the Taliban are not freed, then some of them will probably be killed. Our patience is running out."
In a separate case, a German journalist and his Afghan translator were reported to have been briefly abducted. The news magazine Stern said one of its reporters, who had been on holiday in Afghanistan, may have been taken.
Both were later reported to have been freed and there was confusion about whether anyone had been forcibly seized.
· A French soldier was killed in a rocket attack on an Afghan army base southwest of Kabul, the French defence ministry said today. The attack occurred in the province of Wardak where the non-commissioned officer was training a unit of the Afghan army. France has around 1,100 soldiers in Afghanistan, mainly in the Kabul region.
Afghanistan police have recovered the bullet-ridden body of a South Korean evangelical priest, who was among 23 aid-workers held hostage by the Taliban. Earlier reports that at least 8 other hostages were released are now being denied.

"I can confirm that one of the hostages has been killed by the Taliban," said head of the Afghan delegation in charge of the hostage negotiations, Waheedullah Mujadadi who also added that he could not confirm the release of the other 8 hostages. 

"Since Kabul's administration did not listen to our demand and did not free our prisoners, the Taliban shot dead a male Korean hostage,"  told Reuters via telephone. "If the administration of Kabul is not ready to release our hostages, then by 1.00 a.m. the rest of the hostages will be killed," he continued. "That time is the last deadline." Yousuf said the hostage was killed in  district of . Reports claim he was shot ten times.

''Associated Press'' cited two Western officials when reporting that some of the 23 hostages had been released. One of the officials, who spoke on the condition of anonymity, said six women and two men had been taken to the main United States base in Ghazni.

"Talks continue but no one has been freed so far,” said Ghazni governor Mirajuddin Pattan. Taliban spokesperson Yousuf said the claim of a release was "government propaganda."