President Hamid Karzai on Thursday said he wanted to "correct" rather than damage U.S.-Afghan relations, after a storm of protest over his recent anti-U.S. remarks.

Karzai, who on Sunday suggested that the United States was working in concert with Taliban militants, acknowledged there were serious bilateral strains as the NATO-led military coalition starts to withdraw from Afghanistan.

The U.S. commander in Afghanistan has warned troops that they face an increased threat of attack after a series of inflammatory anti-U.S. comments by President Hamid Karzai.

NATO's International Assistance Security Force (ISAF) on Thursday confirmed the contents of a strongly-worded advisory sent by U.S. General Joseph Dunford to his senior commanders on Wednesday.

Afghan President Hamid Karzai Sunday cancelled a high-profile press conference in Kabul with new US Defense Secretary Chuck Hagel, with U.S. officials saying the event was dropped for "security concerns".

A palace official, who declined to be named, said however that the press conference had been cancelled because of "scheduling pressure", without giving further details.

Afghan President Hamid Karzai on Sunday criticized the Taliban for holding daily talks with the United States as they also launch suicide attacks that kill civilians and children.

The Islamist militants deny re-opening talks after they broke off tentative contact with the U.S. in Qatar in March last year due to failed attempts to negotiate a prisoner exchange.

The head of the NATO-led force in Afghanistan has vowed that the long-delayed transfer of detainees from U.S. to Afghan control will not take place if they pose a threat to international troops.

The transfer of a final group of detainees at Bagram jail has been a cause of friction between President Hamid Karzai and the coalition, and a handover ceremony was abruptly canceled on Saturday.

NATO chief Anders Fogh Rasmussen called on the Afghan government Monday to ensure free and fair presidential elections next year after widespread allegations of fraud marred the 2009 poll.

The credibility of the vote is seen as crucial to avoiding an escalation of violence as NATO combat troops withdraw next year, leaving Afghan soldiers and police to tackle the Taliban insurgency.

Afghan President Hamid Karzai on Sunday condemned a NATO helicopter strike in which two brothers, both under seven years old, were shot dead after being mistaken for Taliban insurgents.

The two boys were tending livestock and collecting firewood in the southern province of Uruzgan when they were killed on Thursday in an incident that drew an abject apology from the NATO-led coalition.

Australian soldiers in southern Afghanistan shot dead two children tending cattle, local officials said on Saturday as the international coalition launched an inquiry into the incident.

Civilian casualties caused by NATO-led troops have been one of the most contentious issues in the campaign against Taliban insurgents, fueling public anger and often triggering criticism from President Hamid Karzai.

Afghan President Hamid Karzai on Sunday ordered U.S. special forces out of a central province, saying Afghans working with them had committed torture and murder.

Karzai said the special forces would have to withdraw from Wardak within two weeks as armed Afghan groups they had set up were fueling "insecurity and instability".

The commander of U.S.-led forces in Afghanistan said Sunday he would comply with President Hamid Karzai's order banning Afghan security forces from seeking NATO air support.

Karzai said on Saturday that he would issue a decree ordering an end to local security forces calling in NATO air strikes amid new tensions over civilian casualties caused by such attacks.