However, Lieutenant General Karl Eikenberry, the commander of US forces in Afghanistan, would not predict how many American troops might come home.
"If Nato does move down to the south, clearly I can expect with the adjustment of forces there could be less US presence in that region," Eikenberry said.
He said there are now about 18,000 US troops in Afghanistan as part of Operation Enduring Freedom.
Alliance foreign ministers agreed earlier on Thursday in Brussels on a plan to expand the Nato-led International Security Assistance Force (ISAF) into the south next year.
The move is part of a transition that could ultimately bring the US and ISAF forces under Nato's command.
The general said the Nato troops to be deployed in the south will have "sufficient rules of engagement to vigorously fight the threat that exists in that area".
He said the US would contribute to the Nato contingent and maintain an army aviation force in southern Zaboul province.
The US-led force and ISAF have had separate roles in Afghanistan in the years since the Taliban was ousted in 2001.
"If Nato does move down to the south, clearly I can expect with the adjustment of forces there could be less US presence in that region"
US commander in Afghanistan
American-led combat forces have been fighting armed opponents in the south and along the country's volatile eastern border with Pakistan, while ISAF has kept the peace in Kabul and gradually expanded its reach to less-troubled regions in the north and west.
The latest expansion will add 6000 Nato troops to the 9500-strong ISAF.
It comes amid an upsurge of violence as US and Afghan forces press deeper into remote areas where sympathy for the Taliban remains strong.
Eikenberry said militants in Afghanistan were receiving foreign funding, but he said the US military has found no direct links to Iraq.
"We have no concrete evidence about, let's say, fighters or facilitators moving from Iraq into Afghanistan and conducting direct training of the Taliban forces or the associated movements of Al-Qaida," he said.