The expansion of the Nato-led International Security Assistance Force (ISAF) into the volatile southern part of Afghanistan "will of course lead to the arrival of certainly a few thousand extra Nato forces," he told reporters on Thursday.
"I can't give you the exact numbers, but you may say it will be well over 10,000, between 13 and 14, 15,000," he said, referring to the eventual size of the expanded force.
The ISAF force has been in Afghanistan since late 2001, shortly after the toppling of the Taliban regime, and came under Nato control in 2003.
It currently numbers about 10,000 troops, including around 2000 sent to reinforce security for last month's parliamentary elections who are due to leave again at the end of the month.
The force ensures security in the capital Kabul and across parts of northern and western provinces, but it is due to move into the southern part of Afghanistan next year.
A 20,000-strong US-led contingent is in the south to tackle a rising insurgency by the Taliban and to hunt al-Qaeda chief Osama bin Laden. This has been the bloodiest year in Afghanistan since 2001.
"I can't give you the exact numbers, but you may say it will be well over 10,000, between 13 and 14, 15,000"
Jaap de Hoop Scheffer, Nato secretary-general
De Hoop Scheffer and Nato ambassadors were in Afghanistan for talks on finalising the planned expansion of ISAF and how it would work with the US-led coalition.
The secretary general downplayed reports that some of Nato's 26 members were concerned about the planned expansion, with ISAF troops currently not involved in counter-insurgency operations unlike their US-led counterparts.
Nato members would discuss the planned expansion at a meeting in London around the end of January, he said.
Key issues to be finalised are the command structure of the new operation and its rules of engagement, he said, admitting the situation "might be less benign than in the northern and the western part of the country".
"At the end of day, of course, it will be one operation in one country led by Nato," De Hoop Scheffer said.
"But we are not in that stage, we are in the stage that we have an ISAF operation, we are in the stage that we have Operation Enduring Freedom, a coalition operation," he said.
"As Nato expands in the south... we have to find more synergy between the two and we have to find a command structure which doesn't bring them into conflict with each other."
Nato-led peacekeepers played a vital role in ensuring security for the parliamentary polls, results of which will start coming on Thursday, and the October 2004 presidential election.
British Defence Secretary John Reid said on Tuesday that his country, a key Nato member, was to send extra troops to Afghanistan; but the precise number has yet to be decided.
Speaking after visiting Afghanistan, he said that a previously reported figure of 4000 extra British soldiers was incorrect.
Britain currently has around 900 soldiers there. "But it is right that we will be sending further troops, because we've always agreed we will take over the headquarters from the Italians next year of the ISAF mission there," he told the BBC.