US Defense Secretary Robert Gates said on Thursday it was too soon to set a timeline for shifting security duties from NATO-led troops to Afghan forces, as proposed by Britain.
"I think I would rather have those on the ground in Afghanistan make the judgment call about when a province or a district was ready to be turned over, rather than specific dates," Gates told a news conference.
Asked about British Prime Minister Gordon Brown's proposal for a timetable for a gradual security handover from 2010, Gates said: "I think it's too early to say."
A handover could occur in some Afghan provinces and districts "relatively soon" and Washington and its allies wanted to see Kabul take the lead for security "as soon as possible," Gates said.
"But it would be counterproductive to transfer that responsibility before the Afghans were ready and had the capacity to sustain the security when we turn it over."
Gates cited Iraq as a model, where US forces have gradually transfered security responsibility to Baghdad government forces over the past two years.
"In my mind I see this happening very much along the lines that we saw in Iraq, where we partner, then we pull back to a tactical overwatch situation, and then a strategic overwatch situation, as the local security forces -- meaning Afghan or Iraqi -- take increasing responsibility."
He added that once the tide turned against insurgents in Iraq, security began to improve rapidly.
"You know, things turned pretty quickly in Iraq once they started to turn. So I think we just have to wait and see."
The transfer in Iraq occurred after former president George W Bush ordered a "surge" of additional US forces in 2007 to bolster security.
Gates' comments came as President Barack Obama prepares to announce his decision on whether to endorse a major troop buildup and after Afghan President Hamid Karzai promised his government's forces would be in charge of security within five years.
Karzai said on Thursday at his swearing in for a second presidential term that Afghan troops would soon be taking the lead, allowing the 100,000-strong international force to scale back.
"We are determined that within the next five years the Afghan forces are capable of taking the lead in ensuring security and stability across the country," he said.
"The role of international troops will be gradually reduced and limited to support and training of Afghan forces."
On Monday, the British prime minister suggested a timeframe for handing over security district by district -- starting as early as 2010 -- could be drawn up in the new year at a conference in January.
Echoing Brown's comments, NATO Secretary General Anders Fogh Rasmussen said on Tuesday that the alliance should begin handing over security duties next year.
"We will do this in a coordinated way, where conditions permit, and this will allow us to progressively move into a support role," Rasmussen said.
The NATO chief said he expected an increase in international forces to be agreed "in a few weeks" but underlined that the ultimate goal was to move from a combat to a support role, starting next year.
As the war enters its ninth year, NATO countries are struggling to rally public support for the mission amid rising casualties and doubts about the legitimacy of Karzai's government.
On the same day Karzai was inaugurated, bombers killed two US soldiers and 10 Afghan civilians in separate attacks in the south of the country, part of Karzai's ethnic Pashtun heartland.