ABOARD A U.S. MILITARY AIRCRAFT - The United States cannot wait for problems surrounding the legitimacy of the Afghan government to be resolved before making a decision on troops, U.S. Secretary of Defense Robert Gates said.

Gates, speaking to reporters on board a plane traveling to Tokyo, described the situation in Afghanistan as an evolutionary process that would not improve dramatically overnight, regardless of what course is taken following the country's flawed August election.

I see this as a process, not something that's going to happen all of the sudden, Gates said.

I believe that (U.S. President Barack Obama) will have to make his decisions in the context of that evolutionary process.

On Sunday White House Chief of Staff Rahm Emanuel had said Obama's Afghanistan strategy and pending decision on additional U.S. troops depend on whether the Kabul government is effective.

The overriding question is not, how many troops you send, but do you have a credible Afghan partner, Emanuel said, adding it was important the election outcome be seen as legitimate and credible.

International observers have called for an election run-off after a U.N.-backed fraud watchdog invalidated tens of thousands of votes for Afghan President Hamid Karzai from the August poll.

Pentagon press secretary Geoff Morrell said Gates believed the issue of the Afghan government's legitimacy went well beyond the question of whoever would be declared winner of the election, or an eventual run-off.

It depended on whether the government in Kabul had the faith and confidence of the people, Morrell said.

Another vote could be complicated by a strengthening Taliban insurgency that has tied up tens of thousands of foreign and Afghan troops and the approaching harsh Afghan winter that cuts off hundreds of villages every year.

Gates said he was confident U.S. and NATO forces could provide security for a run-off, should one be decided, but added bad winter weather could prevent Afghans from voting.

I think the key consideration before us at this point is actually less (one of) security ... (it's) the weather. So getting something done before winter sets in will be very important, he said.

FADING PUBLIC SUPPORT

Gates did not say when he expected U.S. President Barack Obama to decide on whether to increase troops, a decision complicated by rising casualties and fading public support for the stalled, eight-year-old war.

But he pointed out that further high-level deliberations would need to wait for the return of cabinet members from foreign travels through part of next week.

It's just a matter now of getting the time with the president when we can sort through these options and then tee them up for him to make a decision, Gates said.

Gates is traveling this week to Tokyo, Seoul and then to a NATO conference in Bratislava and said he would urge allies on all these visits to contribute to the Afghan campaign.

The top U.S. and NATO commander in Afghanistan, Gen. Stanley McChrystal, said in a classified assessment leaked to the media that the Afghan mission risked failure without additional troops.

Gates said he did not need to wait for a U.S. decision on troops to speak to NATO allies about McChrystal's resource request, saying the reality is that this is an alliance issue.

We ought to do this in a way that if General McChrystal has a set of needs, it should not be looked upon as exclusively the responsibility of the United States to respond, he said.

So I think that having a discussion of that and the fact that this is a continuing shared responsibility makes it entirely appropriate to have that conversation in Bratislava, before decisions are made by the United States.

(Editing by Sanjeev Miglani and Jerry Norton)

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