At a meeting of NATO defence ministers, the allies also agreed to deploy a training mission in Iraq before the end of the year, NATO and US officials said.
On Afghanistan NATO chief Jaap de Hoop Scheffer said after the talks that he would ask military planners to draft options for integrating the International Security Assistance Force and the US-led Operation Enduring Freedom.
"I think the options the military authorities are going to present to ministers in February will certainly include as an option the possibility of a unified command," he said.
Germany in particular is constitutionally obliged to avoid being drawn into frontline combat missions like those carried out by US troops in southern Afghanistan, as opposed to the general peacekeeping role of ISAF.
Berlin's Defense Minister Peter Struck said the mandate of the ISAF force in Afghanistan was to stabilize the country, not to fight international terrorism. "Therefore we are against a merger of the two mandates," he said.
But US officials -- who insist that the Afghan merger plans are not part of efforts to allow US troops to pull out of Afghanistan -- welcomed the outcome of talks in the plush Romanian ski resort of Poiana Brasov.
"It just makes sense ... to see if we can increase the synergies, the interaction and eventually the integration between these two forces," said US ambassador to the North Atlantic Treaty Organization (NATO) Nicholas Burns.
Burns has suggested that the integration of the US and NATO forces could begin as early as next year but acknowledged it was a "very difficult issue" that will require months of discussion.
"But it is a very good first step," he said.
The United States has nearly 20,000 troops in Afghanistan. They are still seeking to pacify the country's southeastern border regions three years after the fall of the hardline Islamic Taliban regime after a US-led invasion.
Rumsfeld first floated the idea of having NATO taking over all military operations in Afghanistan in December 2003. But the idea has until now remained in the background while NATO struggles to expand its Afghan operation.
NATO completed the first phase of an expansion outside Kabul to relatively calm areas in northern Afghanistan just before October 9 presidential election. But a planned expansion into the west is behind schedule.
Key US ally Britain meanwhile said that a tie-up of the two forces should be possible while respecting the views of countries like Germany which wants to avoid being drawn into frontline combat operations.
"I don't see any reason why we should cross any German red lines or indeed red lines from other nations," said defence minister Geoff Hoon.
But France also immediately set out its objections. "I don't believe today in a single command for the two operations," said French Defence Minister Michele Alliot-Marie.
On Iraq, the United States said its NATO partners agreed to have NATO trainers on the ground in Iraq before the end of the year and ahead of January polls, which have been threatened by an increasingly violent insurgency.
"I think what it means is we are not going to wait for the full complement (of trainers)," Burns said. "As soon as those trainers are available, and we identify them from the allied countries, including our own country, they will go in."
NATO's agreement last week to expand the training mission in Iraq deepens its involvement in the violence-wracked country despite bitter opposition from Germany and France to the US invasion last year.
Under the plan some 300-400 officers will train senior Iraqi military officers at a military academy on the eastern outskirts of Baghdad, according to NATO officials. The NATO presence could involve up to 3,000 troops when support personnel are added.
Britain's Hoon said the NATO mission was an important part of US-led forces' exit strategy from the war-scarred country.
"The exit strategy is dependent on the ability of the Iraqis to organize their own security. The sooner that happens ... the sooner it will be possible for foreign forces to leave Iraq," he said.