Afghanistan's presidential spokesman on Tuesday described the NATO-led military operation in the war-torn nation as "aimless and unwise", in the latest broadside by Kabul against the international coalition.

Aimal Faizi, spokesman for President Hamid Karzai, hit out after NATO chief Anders Fogh Rasmussen rejected Karzai's recent allegations of US collusion with the Taliban.

"The people of Afghanistan ask NATO to define the purpose and aim of the so-called war on terror.... (They) consider this war as aimless and unwise to continue," Faizi said in a statement.

The verbal onslaught is likely to worsen relations between Afghanistan and the international coalition that has been fighting for 11 years against Islamist militants who are trying to overthrow Karzai's government.

In recent weeks the Afghan leader has been staking out increasingly nationalist ground as he prepares for his final year in office before he steps down when presidential elections are held in April 2014.

Next year will also see the withdrawal of all NATO combat troops from Afghanistan, leaving poorly trained and inexperienced local security forces to take on the insurgents.

Rasmussen, at a press conference on Monday in Brussels, said that instead of alleging collusion with the Taliban, Afghanistan should acknowledge NATO efforts to bring progress to the country.

"I reject the idea... there is so-called collusion between NATO forces... and the Taliban. It is an absolutely ridiculous idea," he said.

"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 added.

Kabul's angry response deepens a war of words that threatens to derail NATO attempts to ensure a smooth security transition as international troops head home and fears grow that Afghanistan could tip into further instability.

Faizi said that Afghan people "question why after a decade, this war in their country has failed to achieve its stated goals, but rather has resulted in the loss of thousands of innocent lives".

In a reference to neighbouring Pakistan, he added: "It is clearly known to NATO that terrorism sanctuaries are outside Afghanistan, why this war then continues in (Afghan) homes and villages unproductively?"

The United States, which provides 66,000 of the 100,000 NATO-led troops in Afghanistan, was stunned by the accusations earlier this month from Karzai, who accused the US of colluding with the Taliban to justify its presence in the country.

Many analysts say that Karzai, who rose to power with US support after the fall of the Taliban regime in 2001, is desperate to shake off allegations that he is a "puppet" president controlled by foreigners.

In a series of recent moves he demanded the US hand over suspected militants, banned US special forces from the strategic province of Wardak over alleged abuses and stopped local forces from calling in US air support.

The White House has rejected Karzai's allegations, while General Joseph Dunford, the US commander of NATO forces in Afghanistan, last week admitted that they were "at a rough point in the relationship" with Karzai's government.

The United States on Tuesday warned Afghanistan that ensuring a credible presidential election next year would be "critical" to maintaining international support after 2014.

"An inclusive and credible presidential election in 2014 is critical for the country's future and to sustaining international assistance to the people of Afghanistan," US ambassador to the United Nations Susan Rice told a UN Security Council meeting in New York.

"Broad participation and a credible process are essential to reaching the goal of a widely accepted leadership transition," said UN leader Ban Ki-moon.