Nato foreign ministers meeting in Brussels have endorsed a plan to expand the alliance's role in Afghanistan.
Nato forces have been largely kept out of combat
It will involve deploying 6,000 more troops in the south of the country, a third of them expected to be British.
Thursday's agreement is set to make Nato's Afghanistan mission its biggest ever operation outside Europe.
The south and east have been the scene of intense violence which has this year left more than 1,400 dead, making it the deadliest year since 2001.
BBC defence correspondent Rob Watson says that some member states have been worried about potential casualties among their troops, which has made reaching Thursday's agreement difficult.
"We have today agreed to move Nato's support for peace and security in Afghanistan to a new level," Nato foreign ministers said in a statement.
The agreement provides for a new British-led Nato headquarters in Kabul.
Our correspondent says that to ease the concerns of some member states, it is being stressed that the troops' mission will be to promote peace and stability, and not to hunt down members of al-Qaeda and the Taleban.
That will remain the job of the US-led operation Enduring Freedom.
Britain, Canada and the Netherlands will lead the expansion of the alliance's forces to the south of Afghanistan but more troop contributions will be needed before the plan is implemented.
"We are committed to stay the course," Nato Secretary General Jaap de Hoop Scheffer told ministers.
"On the basis of this operational plan which ministers endorsed today, next year, Nato then will be operating in three-quarters of the territory of Afghanistan, will have several thousand more forces, more troops under Nato command, and as you know, Nato will move into the south of the country.
"But we are not working in a void there. Other international actors should stay equally committed."
US Secretary of State Condoleezza Rice described the new agreement as a "long-term plan of security co-operation" for Afghanistan to "help reform and strengthen its defence institutions".
She said: "We will do this of course in partnership with the Afghan government."
However the BBC's Andrew North in Kabul says it is still not clear how the command structure between Nato and the US-led forces will work.