UK Defence Secretary Dr Liam Fox and his US counterpart Robert Gates have reinforced their commitment to Afghanistan after talks in London.
Mr Gates said more civilian support would be provided from July 2011, as military withdrawal begins - "conditions on the ground" allowing.
UK troops would stay until Afghanistan was "stable enough" to ensure internal and foreign security, Dr Fox said.
The pair also expressed optimism in UN moves on Iran's nuclear development.
Mr Gates said he was hopeful the United Nations' security council would pass a resolution designed to prevent Iran developing nuclear weapons "very soon".
Prime Minister David Cameron had met Mr Gates in Downing Street on Monday and restated his support for American strategy in Afghanistan.
On Tuesday, Mr Fox said the first question he had asked himself when he took on his role last month was whether British troops should be in Afghanistan, given the human cost.
"I still believe there's a national security imperative," he said.
"We cannot allow Afghanistan to lapse back into a failed state which will create a security vacuum, which will contaminate the region and possibly well beyond it.
"We are committed to seeing it through to a resolution. By that I mean creating a stable enough Afghanistan to manage its own internal and external security."
Nato commanders had discussed the possibility of moving the UK's 8,000 forces in Helmand to neighbouring Kandahar province when Canada pulls out its contingent there next year.
But Dr Fox said today it was "highly unlikely" this would happen.
"It is certainly not something that we will be proposing," he said.
"We have developed an expertise in understanding the terrain. We understand the personalities in terms of the politics and the governance issues."
However, he said discussions had taken place over whether more US troops would be needed.
Mr Gates said next July would mark the beginning of a "process of transition" which would see a less military and more "developmental" relationship, aimed at building the Afghan economy.
"How fast that proceeds will depend on conditions on the ground, as will the pacing of any withdrawals of alliance forces," he said.
However, he said convincing the Afghan population they would not be abandoned was key.
"We intend, as an alliance… to be Afghanistan's partner for a very long time to come," said Mr Gates.
It was announced earlier that a soldier from the 3rd Regiment Royal Horse Artillery, attached to 4th Regiment Royal Artillery, had become the 293rd member of the UK forces to die in Afghanistan.
Mr Gates offered condolences to the families of British service personnel who have died in the conflict since 2001, saying he was "in awe of their valour".
Mr Gates praised the UK's "leadership" in Afghanistan and spoke of the value of US-UK relations.
The countries had "endured many shared sacrifices" to defend the free world from "dictators, tyrants and virulent ideology", he said.
"I'm confident this special relationship will be, as it has in the past, the bedrock alliance of partners guaranteeing a peaceful and prosperous future," he added.