The Islamist rebels have demanded that both countries pull their troops out of the war-torn country where the Germans serve under Nato and the Koreas under US-led command, a spokesman said.

Taliban spokesman Yousuf Ahmadi, speaking by telephone from an undisclosed location, also said the group welcomed South Korea's decision to pull its troops out of the country by the end of the year, a previously planned withdrawal reiterated in Seoul Saturday.

"We'll decide the fate of the South Koreans later," he said. "But we welcome their government's decision to pull out of our country. I hope their (hostage) problem is solved peacefully."

Earlier in the morning, the Taliban spokesman issued two tight deadlines, threatening the killings of the Germans if the group received no calls from the governments in Berlin or Kabul.

He later announced the killings, which could not be independently verified.

"We executed one of the Germans and will kill the other one unless the government of Germany or the Afghan government contact us for negotiations," he said after a noon (0730 GMT) deadline passed. He added that the German was killed by a bullet.

After the second deadline had passed an hour later, he said in another phone call: "Since the governments did not contact us, we killed the second German hostage at 1:10 pm."

The rebel spokesman offered no evidence but said the Taliban would consider how to hand the bodies over to German or Afghan authorities "if they ask for them," adding: "Otherwise we'll dump them to somewhere."

Germany's Foreign Ministry said it was taking "very seriously" the Taliban's claims but had no independent confirmation.

"We are taking these declarations very seriously and are following the situation closely. But so far we have no independent confirmation that a kidnapped German has been killed," ministry spokesman Martin Jaeger.

The two Germans, whose identities have not been revealed, were kidnapped Wednesday along with five Afghans, provincial officials and police said.

A day later militants abducted the 18 South Koreans, evangelical Christian aid workers aged in their 20s and 30s, the biggest group of foreigners to be kidnapped in Afghanistan since the fall of the Taliban regime in 2001.

Both kidnappings took place on the Kabul-Kandahar highway leading to the insurgency-wracked south.

South Korea's President Roh Moo-Hyun, speaking before the reported killings, urged the immediate release of his country's nationals, as Seoul said it would withdraw troops from the war-torn country by the end of 2007.

"The kidnappers must return the South Koreans unharmed at the earliest possible date," Roh said in a speech on national television. "Under no circumstances should there be any loss of life."

The Koreans left on July 13 for the staunchly Islamic country to "do evangelical activity and volunteer work" in hospitals and children's homes and were due to return home Monday, their church said.

Foreign Minister Song Min-Soon said earlier: "Under the existing plan, we have only several months to go before the troops complete their mission and pull out "

But he stressed: "They can't simply pack up and leave immediately. They move as dictated by the military plan."

Roh later phoned Afghanistan President Hamid Karzai and called for Kabul's cooperation. Song also said a crisis team from Seoul would arrive in Kabul Saturday to help secure the release of the hostages.

A foreign ministry official said 340 Koreans -- 200 soldiers and 140 civilians -- are known to be in Afghanistan, but said it had no immediate figures on short-term visitors such as the hostages.

The Islamist hardline Taliban, ousted from power nearly six years ago by a US offensive after the 9/11 attacks in the United States, are waging an insurgency against the American-backed government of President Karzai.