KABUL, July 22 (Reuters) - A South Korean government delegation arrived in Afghanistan on Sunday to try to secure the release of 23 of their countrymen kidnapped by Taliban insurgents demanding Seoul pull its troops out of the country.

Taliban spokesman Qari Mohammad Yousuf said insurgents would start killing the hostages if South Korea did not agree to withdraw its 200 military engineers and medics by 1430 GMT on Sunday and the Afghan government did not free Taliban prisoners.

The South Korean government has said it will withdraw its troops at the end of this year as planned.

The delegation of eight South Korean officials, including a deputy foreign minister, a special advisor to the president and Foreign Ministry diplomats, was to meet Afghan officials throughout the day. It was unclear if they would make any attempt to contact the kidnappers and try to negotiate their release.

An Afghan Ministry of Defence spokesman denied any operation had been launched to rescue the hostages. "The operation has not begun yet," said ministry spokesman Zahir Murad.

Referring to an earlier Ministry of Defence statement which said a joint operation was underway with international forces, Murad said: "It must have been a computer error."

The governor of the province of Ghazni where the Koreans were seized on Thursday said: "There is no operation, it is rubbish."

U.S. and NATO-led forces in Afghanistan also said they had no knowledge of any such operation.

Taliban spokesmen Yousuf said fighters were holding the captives at different locations and any attempt to free them with force would put the Koreans' lives at risk.

URGENT MISSION

The South Korean mission was given added urgency after the Taliban spokesman said militants had killed two German hostages on Saturday after Berlin refused to yield to demands for it to pull its troops out Afghanistan.

German authorities have cast doubt on the authenticity of the Taliban spokesman and Foreign Minister Frank-Walter Steinmeier said analysis suggested one of the German hostages was alive while the other had died of "stress and strain".

The online edition of German weekly Der Spiegel said the dead German hostage, identified Ruediger B., was diabetic and died after his kidnappers failed in their attempts to get him the necessary medications through intermediaries.

The police chief of Wardak province, north of Ghazni, Mohammad Hewas Mazlum denied media reports quoting him as saying that the body of one of the Germans had been found.

"I have not said to anyone that the body has been found. This is wrong," he told Reuters.

The 23 Koreans belong to the "Saemmul Church" in Bundang, a city on the outskirts of the South Korean capital, Seoul.

Most of them are in their 20s and 30s and include nurses and English teachers. South Korean President Roh Moo-hyun said on Saturday the Koreans were providing only free medical or educational services with no missionary intentions.

The Koreans are the biggest group of foreigners kidnapped so far in the Taliban campaign to oust the Western-backed government and force out foreign troops.

Tearful relatives prayed for their safe release at their church on Sunday.

"My kids went to the war-ravaged country to do volunteer work, carrying love," said Seo Jung-bae, 57, whose son and daughter were both taken hostage. "I feel like chopping off my foot for letting you go. I hope you will return to us and the country without a single hair damaged."

The area south of Kabul where the Germans and Koreans were seized this week has seen a marked escalation of violence in the last month as Taliban militants have moved in from the south.

Residents say government troops only hold the major towns and much of the countryside is beyond their control.

(Additional reporting by Soyoung Kim in Seoul)