A NATO statement said the alliance's council of 26 member states gave final authorization on Friday for the expansion into six provinces of the south of the country.

"This is NATO delivering on its commitment to Afghanistan," said alliance spokesman James Appathurai.

The handover will bring the number of troops under NATO command in Afghanistan to about 18,000 from 10,500 and comes at a time when the country is going through its bloodiest phase since the Taliban's overthrow.

More than 1,700 people have died in militant-linked violence this year. This has intensified in the build-up to the NATO takeover in what analysts have seen as a guerrilla attempt to discourage European troop deployments.

U.S. troops from the multinational Operation Enduring Freedom (OEF) have borne the brunt of fighting against Taliban guerrillas and their Islamist allies in southern Afghanistan since driving the militants from power in late 2001.

More of that task will be undertaken by British, Canadian and Dutch troops, who will come under NATO command but still be able to call on air and other support from the U.S.-led force.

OEF will continue to hunt militant leaders, who include al Qaeda chief Osama bin Laden and Taliban leader Mullah Mohammad Omar, while the primary role of NATO forces will be to create conditions needed for reconstruction and development.

The International Security Assistance Force NATO leads had previously been operating only in the capital, Kabul, and the safer north and west of the country.

The NATO force in the south will have a "robust" role, Appathurai said.

"They can defend themselves aggressively and if necessary pre-emptively. But their mission is not simply to go out and hunt the terrorist leadership -- that is the coalition's role."

The NATO expansion comes after the United States pushed for a bigger European involvement in the battle against militants, but Washington says any reduction in U.S. troop numbers will depend on security conditions.

Six British soldiers have been killed since June and on Thursday officials said two Dutch soldiers were among at least 12 people who died when a helicopter crashed in southeastern Afghanistan, the first Dutch military fatalities in the country.