The expansion will consolidate military command under top NATO leader British Lt. Gen. David Richards and phase out the role of U.S. Lt. Gen. Karl Eikenberry, whose troops will be transferred to NATO, said Mark Laity, an alliance spokesman in Kabul.
Of 40,000 foreign troops in Afghanistan, only 8,000 U.S. troops will function outside NATO control: those tracking al Qaeda terrorists or involved in air operations, Laity said. The overall level of American forces will remain around 20,000.
"In a few days, on a date yet to be declared, you will see the completion of the steady expansion of ISAF," the NATO-led International Security Assistance Force, Laity said.
The NATO expansion into the east wasn't expected to happen for a few weeks. The alliance's troops took command of southern Afghanistan just two months ago and have struggled to stem escalating violence.
A forthcoming NATO order will give the exact date of the handover that places 12,000 U.S. troops in eastern Afghanistan headed by U.S. Army Maj. Gen. Benjamin Freakley under alliance command.
The NATO takeover caps an already historic expansion of missions for the largely European alliance that was created as a Cold War bulwark against the Soviet Union. NATO has seen 45 soldiers killed since arriving in August 2003. Its combat role in southern Afghanistan is the largest the alliance has ever undertaken.
Five years ago, Laity said, the alliance would have been unprepared for such a mission. Afghanistan has been billed as a make-or-break mission whose outcome could decide NATO's ultimate survival.
"It is a big deal," he said. "We've got an unprecedentedly complex mission here. In some areas it's heavy combat. In other areas it's law and order. And it includes everything in between."
With NATO handed the lead role, Washington appears to shed some responsibility for a stability project that, at least this year, appears to be headed in the wrong direction.
The Islamist Taliban have staged an unexpected resurgence and stepped up attacks, triggering major battles that have left more than a thousand dead in the past few months. U.S. voters readying for midterm elections have expressed fatigue with the U.S. administration's twin wars in Iraq and Afghanistan.
"I don't believe the Americans are trying to extricate themselves. They're trying to share the burden with their allies," Laity said.
The move leaves Eikenberry's role in doubt. ISAF spokesman Maj. Luke Knittig said Eikenberry may remain in Afghanistan but under the auspices of the U.S. Embassy.
"He won't be in our chain of command," Knittig said.
Eikenberry "may work with the U.S. ambassador. But no decision has been made," said Knittig, a U.S. Army officer with NATO.
The command consolidation under NATO confines direct U.S. control to a single chief enclave: the sprawling American base at Bagram. A U.S. Army helicopter unit based at Kandahar airfield will also remain under American oversight, Laity said.
Knittig said the U.S. Coalition Air Component Command at Al-Udeid air base in Qatar, in the Persian Gulf, will continue to control airpower in Afghanistan, including U.S. aircraft based in Afghanistan as well as Qatar, the United Arab Emirates, Kyrgyzstan and the island of Diego Garcia.
"Air support has never been a formal part of ISAF and it will remain that way," Laity said.
U.S.-operated prisons and interrogation centers at Bagram will remain under U.S. command, while NATO will continue to transfer its detainees to Afghan police.