Karzai was quoted by numerous members of parliament as saying in a meeting Saturday that he would join the Taliban insurgency if the U.S. and its allies continued pressuring him publicly to do more to end graft, cronyism and electoral fraud.
The White House was already infuriated with Karzai for accusations made earlier in the week in a public forum where he alleged the U.N. and the international community interfered in last year's Afghan elections. The United States and the U.N. strenuously denied the allegation.
Karzai's comments fueled questions whether the temperamental leader could be a reliable partner in the escalating war against the Taliban. NATO reported another service member was killed Wednesday by insurgents in southern Afghanistan, the ninth foreign death this month. No further details were released.
The Taliban threat, though not taken seriously, prompted a warning Tuesday from the White House that it would consider canceling Karzai's May 12 visit to Washington.
During his weekly media briefing Wednesday, presidential spokesman Waheed Omar sought to make light of the firestorm surrounding the Taliban remarks, saying he was surprised to see them in print and had no idea where they had come from.
"That was, I think, a funny thing in the media, and we really, we were shocked to see such kind of comment in the media," Omar said.
He said the Afghan government has put fighting against terror and against "those who put the lives of Afghan people in danger" as its top priority, "and in that context, that comment, whoever has come up with that comment, does not make sense."
However, Omar defended Karzai's claims of foreign interference in last year's presidential election, saying the president expressed his views publicly "to correct the record."
"And the president felt ... we have to avoid having the same conversation again in the parliamentary elections," Omar said. "And based on the facts ... he thought there was information on the other side, on the side of the Afghan government, that the public has the right to know. So that's why the president did that."
Omar said the head of Afghanistan's election commission, Azizullah Lodin, was stepping down because he did not want his three-year term, which expired two months ago, extended further "and the president agreed with that."
Lodin and the commission were widely criticized last year for ignoring rampant ballot-stuffing and other fraud, which the separate, U.N.-backed watchdog group uncovered.
Some Afghan officials attributed Karzai's outbursts to his punishing work schedule and the pressures he faces to placate political supporters seeking patronage, domestic critics who fault his leadership, and foreign backers demanding clean government -- all in the middle of a war. The officials spoke on condition of anonymity because they didn't want to talk on the record about Karzai's behavior.
Karzai's speech critical of alleged foreign interference in the August ballot was delivered to election commission employees and may have been triggered by the international pressure to make changes despite his insistence that the Afghan body did a good job last year.
Last week President Barack Obama flew to Kabul for a six-hour visit to press Karzai to step up the pace of reform.
Public pressure to reform Afghanistan's weak government and combat corruption have increased since the Obama administration took office. Karzai had been a favorite of the Bush administration, which helped install him in power after the downfall of Taliban rule in the 2001 U.S.-led invasion.
International criticism of Karzai escalated after the August election, when the U.N.-backed watchdog threw out nearly a third of his votes, forcing him into a humiliating runoff with his top challenger, Abdullah Abdullah. The challenger later dropped out of the race claiming he had no assurances the second ballot would be fairer than the first.
In February, however, Karzai issued a decree expanding his control of the anti-fraud watchdog and removing foreigners from the body. Last week parliament threw out the decree, prompting Karzai to renew his allegations of foreign interference, and to complain the lawmakers were under the influence of foreign embassies.
Also Wednesday, Germany's foreign minister joined the calls for the Afghans to place more emphasis on good governance, saying that was necessary to allow foreign forces to begin withdrawing. Germany has more than 4,000 troops in Afghanistan -- the third largest foreign troop contingent.
"We work well and fairly together with President Karzai and the Afghan government, but we don't want to be in Afghanistan in the long term," Foreign Minister Guido Westerwelle told reporters in Berlin. "So the Afghan government must implement its own program of better governance, particularly in combating corruption."
Meanwhile, a suicide attack Wednesday on a NATO convoy passing through the eastern Afghan city of Jalalabad killed a civilian and wounded 15 others, officials said. One of the vehicles and several nearby shops were damaged.