Charges by Afghanistan President Hamid Karzai of US collusion with the Taliban are "absolutely ridiculous," NATO head Anders Fogh Rasmussen said Monday.

Asked about Karzai s outburst last week, Rasmussen said he was concerned and hoped Kabul would instead acknowledge the effort made by NATO allies in blood and money to "bring progress to Afghanistan.""I reject the idea there is so-called collusion between NATO forces ... and the Taliban. It is an absolutely ridiculous idea," he said, noting that Karzai had later modified some of his comments.

"We respect Afghan sovereignty but we want acknowledgement that we have invested blood and treasure in helping President Karzai s country to move forward," he told a press conference.Karzai will stand down after elections in April 2014, with NATO forces set to end their combat role completely by the end of that year.Karzai has a record of off-the-cuff remarks that critics often say raise tensions.

But others say his words are carefully chosen and speak directly to Afghanistan s young, often illiterate and poorly educated population.Last week s remarks prompted a warning by the US commander of NATO forces in Afghanistan that they could increase the threat of attack from militants and rogue Afghan forces.

"Karzai s remarks could be a catalyst for some to lash out against our forces -- he may also issue orders that put our forces at risk," US General Joseph Dunford said in an advisory reported by The New York Times at the weekend.

"We re at a rough point in the relationship," Dunford said in his advisory. "(Militants) are also watching and will look for a way to exploit the situation -- they have already ramped up for the spring."