KABUL (Reuters) - The United States must keep fighting the Taliban or risk more attacks like those of September 11, 2001, because the insurgent group is a ruthless enemy that has not cut ties to al Qaeda, the U.S. ambassador to Kabul said.
Ryan Crocker, a career diplomat who was ambassador in Iraq, also warned the United States would have to spend billions more in the coming years to bolster Afghanistan's government and security forces as its own troops prepare to return home.
"What we have to do is I think demonstrate the strategic patience that is necessary to win a long war," he told Reuters, in an interview ahead of the 10th anniversary of the attacks.
"It is going to require more resources, its going to require time. I hope we can bring all those to bear, because as hard, painful, as expensive as this has been in blood and treasure, it has cost a lot less than 9/11 did."
Crocker flew into New York early on the morning of September 11, 2001, and saw the twin towers of the World Trade Centre collapse as he drove into Manhattan after landing.
He has carried his boarding pass from that flight around the world with him, to a decade of senior positions at the heart of the conflicts that followed in the wake of the attacks.
"My life to a significant degree was never the same after 9/11 ... what drives me is what happened that day, and what I saw. And not that I need a reminder, but this is just a small memento of why we are in this fight and why we need to stay in it."
He described building a stable Afghanistan as "the ultimate guarantee that there will not be another 9/11."
After nearly a decade of fighting in Afghanistan the Taliban have greater reach than any time since they were ousted from power, and civilian casualties -- the majority caused by insurgents -- are at the highest since 2001.
"These are tough, determined guys, and we have got to stay in the fight, because if we decide we are done, without completing the mission along the lines I laid out earlier, well the Taliban is going to be back," Crocker said.
Polling showed Afghans do not want the Taliban back, however, and broadly support their own security forces. Western mistakes, especially careless spending, had been corrected, he added.
"I think we all made mistakes, the international community, in the way we put resources into this country. Often without due consultation with Afghan partners, without Afghan buy-in, without appropriate oversight," he said.
"I think we are on the right path now. Yes these are mistakes, but boy the people who are doing the finger pointing ought to come out here and try and get it right in the smoke and dust of a hot war."
Over 1,600 U.S. soldiers have died in Afghanistan, and the war has cost nearly $450 billion, according to the Congressional Research Service. It also stirred up vocal domestic opposition.
Foreign forces have now started handing over control of some areas to the Afghan police and army, and the NATO-led coalition expects to have all combat troops home by the end of 2014.
Crocker hopes this plan will bolster support for the next few years of fighting and institution-building.
"Americans, they are war-weary, it has been a decade, but they also see a plan for future transition. So I think we will be able to maintain the necessary commitments as we move forward to 2014," he said.
The United States is also expected to have some presence in the country beyond that date, with Kabul and Washington currently in trying to hammer out a "strategic partnership" agreement to define the U.S. role longer-term.
Stopping the Taliban fighting their way back to power -- whether with U.S. troops on the ground or through support for Afghan forces -- is critical to U.S. security, Crocker added.
"With the Taliban will come al Qaeda, and we will have the same situation that we had pre-9/11, and that to me is an utterly unacceptable outcome," he said in his Kabul residence, in the heart of the heavily guarded embassy.
"That is a risk of our national security that I think no sane person would willingly take."
Despite preliminary contacts with insurgent groups, Crocker also said he did not expect a negotiated settlement in the short-term, because without stronger military pressure insurgents would not accept changes in Afghanistan, including improvements in women's rights.
"The Taliban needs to be further weakened to the point where they will come to the table prepared to accept the conditions we have set jointly with the Afghans," he said.
"That's not the Taliban I think we are engaged with today."
Crocker dismissed critics who argue that limited progress in Afghanistan is due in part to the shift in focus to Iraq.
"If for example in 2002, 2003 or 2004 we had substantially increased the number of our forces without an active Taliban threat, which didn't come until later, I think there is every chance the Afghans would have seen us as occupiers," he said.
"We could have had a backlash of proportions that would have given us even a worse situation today."
(Editing by Sanjeev Miglani and Ed Lane)