WASHINGTON, July 10 (Reuters) - Afghanistan poses a greater challenge than Iraq for the United States and its allies and will require a commitment from the international community for a generation, Britain's defense secretary said on Thursday.

Des Browne, in the United States to mark the 50th anniversary of a U.S.-British mutual defense agreement, also said the next U.S. administration will need to make NATO's transformation from a Cold War organization a priority to help ensure long-term success in Afghanistan.

"I have no doubt that it will be a longer haul in Afghanistan," Browne said in a speech at the Brookings Institution, a Washington-based think tank.

"The nature and complexity of the challenge there is greater even than the nature and complexity of the challenge in ... Iraq," he said. "It will take a generation to rebuild."

Afghanistan, with a society and infrastructure that has suffered three decades of armed conflict, faces rising violence nearly seven years after U.S.-led forces ousted the country's former Taliban rulers.

Browne said NATO troops have forced the Taliban to abandon its former insurgency methods to adopt new tactics, such as headline-grabbing suicide bomb attacks.

"What this means is that the campaign in Afghanistan can no longer be won by the Taliban," he said.

"However, it can still be lost by the international community if we fail to maintain our cohesion as an alliance and rapidly and sustainably fill the reconstruction and development security space that we have created."

Much of the counterinsurgency burden confronting a 50,000-strong NATO contingent has been shouldered by the American, British, Canadian and Dutch troops, while other NATO members have resisted pressure to operate outside the relatively safe north.

Browne said that experience has made Afghanistan a catalyst for fundamental change in NATO by underlining the need to refocus the organization and its members on new threats, such as Islamist extremism.

"What we need to address this issue more strategically and at the highest possible political level, and what I ask an incoming (U.S.) administration to do, is to support those of us in the NATO alliance who think the transformation of that alliance is its greatest challenge," he said. (Reporting by David Morgan, editing by Philip Barbara)

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