Nato has taken charge of Afghanistan's eastern provinces, which have been under the control of US forces since the Taleban were ousted five years ago.
Afghanistan represents the biggest ground deployment in Nato's history
The alliance's International Security Assistance Force (Isaf) already commands troops in the north, west and south of Afghanistan, as well as Kabul.
Officials say the move will make the force more efficient, as it seeks to secure Afghanistan for reconstruction.
Afghanistan represents the biggest ground deployment in Nato's history.
Some 10,000 troops - mainly US forces - have come under the command of Lt Gen David Richards from the UK.
The addition of US troops brings the total number of troops under Nato command on the ground in Afghanistan to about 31,000.
Afghan President Hamid Karzai, along with the top commanders of the two forces involved, attended a ceremony in Kabul marking the handover.
US commander Lt Gen Karl Eikenberry said this was an "historic day" and paid tribute to the 330 coalition soldiers and 800 Afghan security personnel killed in the past five years.
Gen Richards, promoted to four-star general on Thursday, was bullish about the new force, saying: "If by next spring these improvements are not evident, then I will be surrendering to whoever wants to put me up against a wall."
This expansion was always expected but it is nonetheless something of a vote of confidence in the alliance from the Americans, says BBC defence and security correspondent Rob Watson.
NATO FORCE IN AFGHANISTAN
31,000 troops now on ground in Afghanistan, including 10,000 coalition troops moved under Nato command
37 nations contributing
8,000 US-led troops continue training and counter-terrorism separate from Nato force
*Contribution figures may differ from exact numbers on the ground
But he adds it is happening sooner than planned, which reflects the struggle being faced by UK, Canadian and other Nato troops against a resurgent Taleban in southern Afghanistan - a struggle that has shown the need to pool US forces under Nato command.
UK Prime Minister Tony Blair defended Nato's role in Afghanistan, saying its presence remained "absolutely critical".
"We do not want al-Qaeda and the Taleban back in power in Afghanistan, using it as a training ground for terrorism," he said.
But Mr Blair acknowledged that people in Afghanistan had "suffered" as a result of military action against the Taleban.
His remarks follow a report from the UN refugee agency, the UNHCR, that up to 90,000 people had been displaced by the fighting in southern Afghanistan.
Nato's expanded role brings 14 provinces, including Kunar, Nuristan, Laghman, Nangarhar, Paktia, Paktika and Khost, under its control.
The 37-nation Isaf said the US would retain control of some 8,000 of its troops for their "counter-terrorism" role and for training Afghan police and soldiers.
The responsibility for hunting down al-Qaeda's fugitive figurehead, Osama Bin Laden, and the Taleban will remain with the US forces.
The eastern provinces are considered by many to be an al-Qaeda stronghold.
But Kandahar province MP Khaled Pashtun told the BBC World Service's World Today that although the east had more difficult terrain "the government is more in control [there] than in the south".
Nato's Secretary General Jaap de Hoop Scheffer told the programme the Taleban were concerned now that Nato had entered what was previously "no-man's land".
"Nato brings in twice the force the coalition had in the area, and nobody, be it Taleban, be it drug lords, will appreciate it very much if they see forces coming in and chasing them out."
Nato's supreme commander, Gen James Jones, has said that Nato's exit strategy will depend not on a military victory, but on the successful economic and political reconstruction of Afghanistan.
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