Thursday's move will see Nato troops operating in the volatile east of the country, and their forces faced fierce resistance from Taliban fighters when they entered the south of the country two months ago.
The shift will put between 12,000 and 13,000 US troops under the command of Lieutenant-General David Richardson, the International Security Assistance Force's British commander.
It will mean the largest number of US troops under a foreign commander since World War II, when British General Bernard Montgomery commanded Allied forces in North Africa.
The move was announced at a Nato defence minister's conference in the Slovenian town of Portoroz.
James Appathurai, Nato's spokesman, said the plan to spread Nato's mission showed the success of the operation despite the upsurge of Taliban violence and the alliance has faced difficulties in persuading European nations to provide the extra troops its commanders.
"What it shows is that this operation is moving forward," he said following the meeting.
"I think it demonstrates considerable success."
European ministers are under pressure to send more troops to southern Afghanistan, where soldiers from Canada, Britain, the US and the Netherlands have borne the brunt of the fighting.
Des Browne, the British defence secretary, said before the meeting that he would urge Nato to look again to see what more can be done.
"Allies must step up to the plate to meet our collective commitment to support the government and people of Afghanistan."
Nato also announced a plan to donate surplus equipment to Afghanistan's armed forces, said Jaap de Hoop Scheffer, the Nato secretary-general said.