President Barack Obama's pick for envoy to Kabul warned America could not afford the "disastrous" consequences of walking away from Afghanistan, as debate raged over a pending US troop drawdown.

Ryan Crocker acknowledged at his Senate confirmation hearing Wednesday that making progress in the war would remain "hard" but was not "impossible" despite questions over whether the US commitment to the conflict is sustainable.

But he also said that American aspirations, after nearly 10 years of a grinding war sparked by the September 11 attacks in 2001, must be realistic.

"We're not out to -- clearly -- create a shining city on a hill. That's not going to happen," he said.

The hearing came as Obama discussed a troop drawdown planned for July, which have not yet been finalized, in a video conference with Afghan President Hamid Karzai, and expressed sorrow over recent civilian casualties.

A Senate Foreign Relations Committee report meanwhile cast doubt on the success of the nation-building effort in Afghanistan and warned the fragile country could sink into an economic depression when foreign troops left.

Crocker, a veteran diplomat, credited with helping to pave an exit out of Iraq for US forces as ambassador to Baghdad, admitted before the Senate Foreign Relations panel that improvements in Afghanistan had been hard won.

"It will go on being hard, but hard does not mean impossible," he said, and recalled a recent warning by outgoing Secretary of Defense Robert Gates.

"We walked away from Afghanistan once in 1989, with disastrous consequences. We cannot afford to do so again."

Crocker warned that left unchecked, corruption in Afghanistan could become a second insurgency and said that the US interest was in forging "good enough" governance to prevent a restored haven for terror groups like Al-Qaeda.

Earlier, Obama and Karzai spoke for an hour over secure video link, amid another spike in tensions between Washington and Kabul and as political intrigue mounts over the size of troop withdrawals Obama will order.

"The president expressed his sorrow over tragic civilian casualties, most recently in Helmand province," White House spokesman Jay Carney said.

"Both leaders noted that the Taliban are responsible for the great majority of civilian losses, and agreed that every loss of civilian life is a tragedy and undermines our mission that focuses on protecting the population."

A furious Karzai last week issued a "last warning" to the US military to avoid "arbitrary and unnecessary" operations that kill civilians, after he said 14 people died in an air strike in Helmand province.

The International Security Assistance Force put the death toll at nine and apologized, while saying the strike was carried out after insurgents who had earlier killed a US marine hid in a compound and carried on firing.

Civilian casualties have long been a bone of contention between successive US administrations and Karzai, who is struggling to win hearts and minds of Afghans and deprive the Taliban of propaganda wins.

The United Nations says Afghan civilian deaths in the war increased 15 percent to a record high of 2,777 last year. More than three-quarters of the dead were killed in violence blamed on insurgents.

Carney said Obama and Karzai agreed on "their shared commitment to Afghan-led reconciliation, progress on forging an enduring US-Afghan strategic partnership, and transition to Afghan leadership for security."

Obama has not yet decided on the "pace and the scope" of the drawdown, he said, as competing visions of the size of the future US mission in Afghanistan are aired in the US press as Obama deliberates on next steps.

Gates argues that US-led forces are nearing a "decisive blow" against the Taliban, after 10 years of conflict sparked by the September attacks in 2001.

Reports have long suggested that Obama could make a token withdrawal of around 5,000 troops from the 30,000 strong "surge" force that he sent to the country in December 2009. Around 100,000 US troops are in Afghanistan.

But there is increasing disquiet on Capitol Hill over the human and financial cost of the conflict and the chances of forging a functioning Afghan state.

The Senate Foreign Relations panel report suggested that the planned departure of all US and international troops from Afghanistan in 2014 could spark an economic collapse in the fragile nation.

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