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NATO's military operation in Afghanistan will succeed, the alliance's chief said Tuesday, urging member countries not to lose heart despite a strengthening Taliban insurgency and unexpectedly high casualties.

Speaking to a forum before a two-day summit, NATO Secretary-General Jaap de Hoop Scheffer insisted the alliance will prevail in its first mission outside Europe. He also expressed hopes that by 2008, Afghan forces could begin taking over security tasks.

"I would hope that by 2008, we'll have made considerable progress ... (with) effective and trusted Afghan security forces gradually taking control," he said.

Although De Hoop Scheffer predicted that by 2008, the North Atlantic Treaty Organization also will be able to reduce its presence in Kosovo, where about 17,000 peacekeepers are deployed, he said he could not yet envisage drawing down its 32,800-strong force in Afghanistan.

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"Afghanistan is 'mission possible,"' he said. "We need to be frank about the risks, but we also need to avoid overdramatizing. NATO has been in Afghanistan for three years — time enough to know what it takes to succeed."

Defeating Taliban forces "will require the full commitment of our alliance," U.S. President George W. Bush said Tuesday, calling the Afghanistan mission NATO's No. 1 operation.

"The commanders on the ground must have the resources and flexibility they need to do their jobs," Bush said in a pre-summit speech, crediting the alliance with helping Afghanistan go from "a totalitarian nightmare" to stability and steadily growing prosperity.

The dangers to the NATO force were underscored by recent attacks that have shattered a period of relative calm. Two Canadian soldiers were reported slain by a homicide car bomber Monday. A day earlier, a suicide bomber killed 15 Afghans.

During a stop in Denmark en route to the summit, British Prime Minister Tony Blair stressed that NATO forces must have the flexibility and the troop capacity to go where the need is greatest.

"The important thing is that we recognize the operation in Afghanistan is of crucial importance to our own security. NATO's credibility is at stake here, and if we don't succeed in Afghanistan, the whole of our world will be less secure," Blair added.

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U.S. Gen. James L. Jones, NATO's supreme allied commander, said Tuesday he had commitments waiving up to 15 percent of the restrictions that some nations have imposed on their troops, freeing up some 2,000 soldiers for possible duty in the south.

De Hoop Scheffer called for sweeping reforms to transform NATO into "a major strategic tool for coping with 21st century challenges."

"Partnerships with nations around the world ... hold much potential. The decisions I expect from our summit here should help us unlock this potential," he said.

At the summit, NATO leaders planned to explore the possibility of forging closer ties with Pacific allies.

NATO's emerging new role as a guarantor of peace in global hotspots will be discussed at the summit of 26 presidents and prime ministers, the first such gathering on the territory of the former Soviet Union. Latvia broke away from the Soviet sphere in 1991 and joined NATO in 2004.

Russia is an "important and privileged partner," de Hoop Scheffer said, adding that NATO's relations with Moscow must be "rejuvenated."

Although all 26 allies have troops in the force, Britain, Canada, the United States and others in the front line of the battle in the Taliban's southern heartland have complained that Germany, Italy, Spain and France are keeping their troops in the more peaceful north and west.

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"Other NATO nations have troops there, but have imposed caveats on the use of them and on the use of their equipment — this at a time when NATO's commanders on the ground urgently require additional manpower," said Latvian President Vaira Vike-Freiberga.

"NATO cannot afford to lose this crucial struggle against the regressive forces of a resurgent insurgency by being indecisive or lacking commitment," she said.

France is preparing to assume an expanded role in the Afghan mission, and officials said President Jacques Chirac would propose forming a contact group on Afghanistan to ensure that a global strategy guides NATO action in the country.

"The Europeans have relied on their American allies for too long. They have to shoulder their share of the burden," Chirac said in a statement.