In a claim disputed by Afghanistan and Germany, a purported Taliban spokesman said Saturday that the hard-line militia killed two German hostages but offered to trade 23 captive South Koreans for imprisoned Taliban fighters.

The militant spokesman offered no proof for his claim, and Afghan officials said one of the Germans appeared to have died from a heart attack and that the other was still alive.

"Everything indicates he was a victims of the stress of the kidnapping," German Foreign Minister Frank-Walter Steinmeier said in Berlin.

Qari Yousuf Ahmadi, who claims to speak for the Taliban, said the militants were willing to release the South Korean hostages in exchange for imprisoned Taliban fighters. He said the Afghan and South Korean governments had until Sunday evening to agree to release 23 Taliban militants or the Korean hostages would be killed.

"If the government of Afghanistan and the government of Korea are asking for the release of their hostages, then we believe the Taliban also have the right to ask for the release of their prisoners who are spending time in Afghan jails," Ahmadi told The Associated Press by satellite phone from an undisclosed location.

It is not clear that Afghanistan would agree to such a deal. President Hamid Karzai in March authorized the release of five Taliban prisoners in exchange for a kidnapped Italian reporter, but he called the trade a one-time deal amid criticism from the United States and European capitals that the trade would encourage more kidnappings.

Ahmadi claimed the Germans and five Afghans kidnapped along with them were shot to death because Germany did not withdraw its 3,000 troops from Afghanistan as demanded by the Taliban. The seven were kidnapped on Wednesday in the southern province of Wardak while working on a dam project.

The Afghan government said it had contradictory information.

"The information that we and our security forces have is that one of these two who were kidnapped died of a heart attack," Foreign Ministry spokesman Sultan Ahmad Baheen said. "The second hostage is alive, and we hope that he will be released soon, and we are trying our best to get him released."

He did not reveal the source of the information or say anything about the Afghan hostages.

The South Koreans were kidnapped at gunpoint from a bus in Ghazni province's Qarabagh district on Thursday as they traveled on the main highway from Kabul to the southern city of Kandahar. It was the largest-scale abduction of foreigners since the fall of the Taliban regime in 2001.

The Taliban roam freely through large swaths of southern Afghanistan. The South Koreans' bus was stopped on Highway 1, the main Afghan thoroughfare connecting Kabul and Kandahar. Driving the road is known to be risky, particularly in areas of Ghazni and Zabul provinces where the government has little control.

Ahmadi initially said there were 18 South Korean hostages, but later revised the figure to 23. He said several Koreans spoke the Afghan languages Dari and Pashtu and had been mistaken for Afghans.

Ahmadi also initially said the kidnapped Koreans, including 18 women, would be killed Saturday if South Korea didn't withdraw its 200 troops, which it already plans to do by the end of this year. Late Saturday he changed that demand.

South Korean President Roh Moo-hyun urged the Taliban to "send our people home quickly and safely." Roh also spoke with Afghan President Hamid Karzai and asked for cooperation to quickly win the release of the South Koreans, Roh's office said.

South Korea "is maintaining contact" with the Taliban to try to win their freedom, a senior Korean official said on condition of anonymity, citing the sensitivity of the issue.

U.N. Secretary-General Ban Ki-moon, himself a South Korean, called Karzai and expressed "grave concern" over the abductions, the U.N. said. Ban called on the Afghan government to "do its utmost" to secure the hostages' release.

It was unclear what the Koreans were doing in Afghanistan. The Yonhap news agency reported that most of the hostages were members of the Saemmul Community Church in Bundang, just south of the South Korean capital, Seoul. A year ago, hundreds of South Korean Christians were ordered to leave Afghanistan amid rumors they were proselytizing in the deeply conservative Islamic nation.

Ahmadi warned the Afghan government and U.S. and NATO forces not to try to rescue the hostages, or they would be killed. The provincial police chief in Ghazni province said his forces were working "carefully" to not trigger any retaliatory killings.

"We have surrounded the area but are working very carefully. We don't want them to be killed," said Ali Shah Ahmadzai.

German Foreign Ministry spokesman Martin Jeager, meanwhile, said a crisis team was pursuing "every clue" and was in close contact with the Afghan government.

Germany has 3,000 soldiers in NATO's International Security Assistance Force who are stationed in the mostly peaceful northern part of Afghanistan. South Korea's 200 soldiers in the U.S.-led coalition largely work on humanitarian projects, such as medical assistance and reconstruction.

In South Korea, family members of kidnapped victims urged the government to accept Taliban demands, noting Seoul had already decided to bring home its soldiers by the end of this year.

"We hope that the immediate withdrawal (of troops) is made," Cha Sung-min, a relative of one of the hostages, told reporters.