As the United States starts exiting the Afghan war, analysts warn that dysfunction in Kabul and its naked hostility towards the White House jeopardise the US goal of handing over security in 2014.

The transition to Afghan control is a delicate operation that looks imperilled by the dismal relationship between President Hamid Karzai and Washington, battered and bruised by 10 years of mutual disappointment.

Karzai has increasingly indulged in anti-US tirades, aimed at Afghans weary of another decade of war after Taliban rule, civil war and anti-Soviet jihad, venting anger that US troops do too little to limit civilian casualties.

But the state of the relationship is more critical than ever, with President Barack Obama now counting down to the full withdrawal of US combat troops and the handover to Afghanistan of lead responsibility for fighting the Taliban.

After a decade of limited success, there are just three years left to build up Afghan security forces and ensure that Afghans deliver state services across the board when foreign combat forces are due to leave at the end of 2014.

The Afghan forces will have a big hole to fill. Obama on Wednesday ordered all 33,000 US surge troops home from Afghanistan by next summer and declared the beginning of the end of the war, vowing to turn to nation-building at home.

"It is a reduction but not a significant one and so we at the end of the period will still have 70,000 troops committed to a mission that is not working," former UN envoy to Afghanistan Peter Galbraith told the BBC.

"The Afghan government and military are simply not going to be able to stand on their own because they cannot win the support of the population."

US Defense Secretary Robert Gates has called on Karzai's government to "step up" more than it is doing now to ensure the transition is a success.

But independent experts are scathing, citing weaknesses in the Afghan army and police, which number roughly 300,000 now and are growing rapidly ahead of the foreign withdrawals.

Gilles Dorronsoro of the Carnegie Endowment for International Peace highlighted deep-seated problems in the army including infiltration by Taliban militants, weak training, a large turnover and ethnic imbalances.

"The 2014 transition anticipated by the coalition is unrealistic because the Afghan army will not be capable of containing an insurgency that is gathering significant strength," he said in a recent paper.

Just weeks before the transition is due to start next month in seven areas, the soon-to-depart US ambassador to Kabul, Karl Eikenberry, brought US-Afghan tensions out into the open on Sunday in a rare public broadside against Karzai.

Eikenberry said that comments by Afghan leaders had been "hurtful and inappropriate", adding that when the United States is "called occupiers and worse... we begin to lose our inspiration to carry on".

Last month, Karzai said foreign forces risked becoming an "occupying force". One day before Eikenberry's outburst, the Afghan leader had disclosed for the first time that the United States was holding talks with the Taliban.

Experts say that Eikenberry's successor Ryan Crocker, who was also ambassador in Iraq, faces the crucial job of improving relations with Karzai, whose term in office runs to 2014.

"The arrival of ambassador Crocker represents an opportunity to reset the relationship and define a new way forward," said Jeffrey Dressler, a senior analyst at the US-based Institute for the Study of War.

But the onus rests with Karzai, said Stephen Biddle, a senior fellow at the Washington-based Council on Foreign Relations.

"Almost any time a counter-insurgency succeeds, it's because the host government eventually gives in to outside pressure, grits its teeth and grumbles, and reforms," he said.

"Karzai may yet too... we've made it worse than it needs to be here, however."

Afghan officials shrug off the concerns that their forces are woefully underprepared.

"The Afghan army has gained the capacity to fill the space that will open up in some areas after the withdrawal of these troops," defence ministry spokesman General Mohammad Zahir Azimi said Wednesday.

But security aside, officials acknowledge that a major effort is also required before 2014 to build up Afghanistan's fragile economy and governance.

A senior US official, speaking on condition of anonymity, said that "in their hearts" many Afghans wanted the United States out. But "in their heads, they know the realities on the ground, they don't want us to leave".