But Lt. Gen. David Richards promised that the incoming force would be more "people-friendly" in an effort to win the support of the local population amid rising resentment toward the U.S.-led coalition over its aggressive tactics.
There was no letup in the fighting in the south, meanwhile, with at least 28 people killed, including four in a failed attempt to blow up the Kandahar provincial governor.
Speaking at a news conference in Kabul, Richards, who is British and took command of NATO's International Security Assistance Force a month ago, said the number of troops the coalition has had in southern Afghanistan hasn't been sufficient to deal with the violence.
"They have been relatively short of troops, of boots on the ground," he said.
He said the figure would increase from an average of about 3,000 in recent years to about 6,000 when ISAF assumes responsibility for the volatile region from the coalition next month. He said the new force would also have more attack helicopters.
The changeover comes amid the most intense fighting in the south since the fall of the Taliban in 2001. More than 400 people, mostly militants, have been killed since mid-May, raising fears for this country's future.
Observers in the south say support for the foreign troops has waned in recent months, partly because of a large number of civilians killed mistakenly in coalition operations and also because of an increased presence of Taliban rebels.
President Hamid Karzai has repeatedly complained about the allegedly overly aggressive tactics of the coalition and last year demanded an end to airstrikes and house searches — but the coalition claimed they were essential and did not halt them.
Richards acknowledged that the security situation had deteriorated, but he said he was optimistic his tactics would counter that trend.
"I have a different approach (to the coalition)," he said. "I want to get out much more on our feet in and among the villagers. ... We will gear our security operation around (building) more roads, irrigation, etc."
But he warned that when needed, the NATO force, which has troops from 36 nations, would be tough.
"When we need to be muscular, robust with those opposing us, we have all the capability we need and we will certainly do so," he said.
The U.S. has hiked its troop levels in Afghanistan in recent months from 18,000 to 23,000. It is expected to start to withdraw some of those once ISAF takes over in the south. ISAF's troop levels will jump from the current number of around 9,000 to about 17,000 by next month. The new forces are mainly coming from Britain, Canada and the Netherlands.
The move by NATO to take over in the south is part of a plan for the force to assume control of security across the whole country. U.S. forces after July will still be in charge in the east, but Richards said he was hopeful that area too would soon come under his command.
He said having one force responsible for the entire country would give commanders more freedom to move troops around the country to trouble-spots.
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