KABUL (Reuters) - Taliban insurgents have kidnapped 23 Korean Christians from a bus in Afghanistan, officials said on Friday, the biggest group of foreigners seized so far in the militant campaign to oust the government and its Western backers.

Taliban fighters have increasingly turned away from classic guerrilla warfare and instead have taken up what Afghan officials call "terror tactics" -- kidnapping, suicide attacks and roadside bombs.

Twenty-three Korean citizens, 15 women and five men, were very carelessly traveling in a chartered bus from Kabul to Kandahar yesterday, on the way to Kandahar their bus was stopped by armed men ... and they took them away," said Interior Minister spokesman Zemari Bashari.

He said the incident happened in the Qarabagh district of Ghazni province, some 175 km (110 miles) south of Kabul.

"We are still investigating which organization they were with, and why they were traveling to Kandahar," he said.

The Taliban said they had seized 18 Koreans, 15 men and three women.

"They are safe with us, we are investigating them and our demands and reaction will be announced later." said Taliban spokesman Said Yousuf Ahmadi by telephone from an undisclosed location.

Earlier, local police said the bus was traveling from the southern city of Kandahar to the capital Kabul.

MISSIONARY FEAR

A South Korean Foreign Ministry official said in Seoul about 20 South Korean Christian volunteers were feared to have been kidnapped by Taliban insurgents.

Last year, the South Korean government tried to stop a group of 2,000 Korean Christians traveling to Afghanistan for a peace conference, fearing for their safety.

But 900 of them still came to Afghanistan, causing an uproar in the staunchly Muslim country, where many accused them of being evangelical missionaries, before they were all deported.

Afghanistan's ambassador to South Korea was later sacked for issuing the group with visas, a Foreign Ministry spokesman said.

South Korea has no combat troops in Afghanistan, but has a contingent of 2,200 engineers, doctors and medical staff.

Two Germans and six Afghans were abducted southwest of Kabul on Wednesday and are still missing.

"The German citizens are safe with us. Our demand is the withdrawal of German troops from Afghanistan and also the release of our prisoners," said Taliban spokesman Yousuf Ahmadi.

Germany said it was aware of the Taliban claim.

"We have taken note of the comments from a so-called spokesman for the Taliban. There is also a statement to the contrary from a Taliban spokesman from yesterday. He has indicated that the abducted Germans are not in Taliban hands. There is a contradiction here," a German Foreign Ministry spokesman said in a statement.

"We will carefully and calmly pursue developments. All necessary steps have been taken. The emergency task force is working very intensively on a quick release of both men."

One German national was kidnapped in western Afghanistan this month, but was released unharmed after a few days.

The Taliban kidnapped two French aid workers and three of their Afghan colleagues in southwestern Afghanistan in April, but later released them unharmed.

(Additional reporting by Saeed Achakzai in Spin Boldak and Ismail Sameem in Kandahar)