The so-called operational plan is expected to see British, Canadian and Dutch personnel take leading roles in the NATO push into the south around next May, with 6,000 extra troops being added to the 9,500-strong force.
"NATO will then be operating in three quarters of the territory of
Afghanistan," said alliance Secretary General Jaap de Hoop Scheffer, following talks with the military organisation's ministers in Brussels.
The plan sets out a "double-hatted" command structure under which the NATO-led International Security Force (ISAF) will work more closely with the US-led Operation Enduring Freedom (OEF).
ISAF will set up an extra four provincial reconstruction teams (PRTs), a mixture of troops and civilians helping to spread the Kabul government's rule into the distant provinces, at all points of the compass except the east.
The move has been championed by the United States but resisted by some European countries which fear being dragged into frontline combat operations and away from the peacekeeping role which is the main function of ISAF.
"Of course this expansion will take NATO into more volatile territory, but there should be no doubt... our forces will have the equipment and the support they need for the job," de Hoop Scheffer told reporters.
"They will have the rules of engagement they need to carry out their mission and they will do something very important indeed, they will bring peace to more people in Afghanistan who have suffered terribly," he said.
To fulfil its mission, ISAF's rules of engagement, the political rules governing the use of deadly force, have been beefed up, not just in the south, but across the country.
Dutch Foreign Minister Ben Bot said that the Netherlands had received guarantees that 1,100 of its troops likely to be working in the southern Oruzgan province would have the backing of OEF soldiers in any crisis.
Any prisoners they hand over to Afghan police will also be treated humanely and not face the death penalty.
All ISAF detainees must be released or transferred to the Afghan authorities within four days.
The US has long pushed NATO to take a larger role in Afghanistan, where its own 20,000-strong mission has been fighting insurgents since it ousted the Taliban in 2001 shortly after the September 11 terror attacks.
The Taliban have been waging a guerrilla-style insurgency against Afghan government troops since they were removed from power.
Attacks linked to the insurgency have killed about 1,500 people this year, most of them militants.