(Updates with Brown, Merkel, Sarkozy letter)

KABUL, Sept 9 (Reuters) - Can President Barack Obama ask Americans to send more of their sons and daughters to die in Afghanistan to defend a government willing to steal an election?

That is the stark political question that U.S. officials may have to grapple with in the next few weeks if President Hamid Karzai continues to ignore evidence of fraud in last month's Afghan presidential poll.

An election complaints watchdog mainly appointed by the United Nations has said it had found "clear and convincing evidence of fraud" in the vote and ordered a partial recount.

That did not stop Karzai from praising in a statement on Wednesday the "honest" and "impartial" vote. The disputed preliminary results show him heading for a first round victory.

Diplomats say the election is not over until the recount. Fraud investigators may yet succeed in cleaning up the tally and coming up with a result that Afghans can accept as legitimate, with or without requiring Karzai to face a second round run-off.

"We have to see the result of their investigations," British ambassador Mark Sedwill said. "We always knew there would be fraud in this election, a lot of irregularities, I'm afraid that was inevitable, and we talked about that before the election."

Sorting it out may yet take months. Whatever the outcome, the apparent fraud already on display has further hurt the image of a government seen by many in the West as feeble and corrupt.

Tallies -- some since removed from the election commission's website without explanation -- featured such anomalies as entire villages where Karzai received every single vote cast, including exactly 500 votes each at four neighbouring polling stations.

Karzai's apparent eagerness to ignore concerns, hurry through the process and claim the prize has chilled an already frosty relationship with the new U.S. administration. It could do far more damage in coming weeks when Obama has to decide whether to double down on a risky strategy and send more troops.

WORST TIME

The election standoff could hardly come at a worse time for Obama, who has made Afghanistan the primary foreign policy focus of his presidency. He has already sent 21,000 extra troops to Afghanistan, ramping up an escalation begun at the end of last year by his predecessor George W. Bush.

Since then, the war has become far deadlier, support for it at home has eroded, U.S. opinion-shapers have come out against it and NATO allies have wobbled in public.

Sitting on the president's desk is a week-old classified assessment of the eight-year-old war from Obama's hand-picked new commander, General Stanley McChrystal, which is widely believed to make a case for sending more troops.

Obama was supposed to take that decision after the outcome of Afghanistan's election was clear. Now, an announcement of more troops may coincide with newspaper headlines about a protracted dispute over vote fraud by Washington's ally.

"It will be very difficult to justify the support of the outcome of an election, for which hundreds of millions of dollars have been spent and NATO soldiers have died, ... (if) fraud decides the outcome, not the will of the people," Karzai's main opponent, Abdullah Abdullah, told Reuters.

The relationship between the Obama administration and Karzai -- which both sides have acknowledged got off to an awkward start earlier this year -- now appears headed off the rails.

The day after the election, when Karzai's campaign manager prematurely declared a first-round victory by a wide margin, Obama's envoy Richard Holbrooke had a tense meeting with Karzai to urge him not to pre-empt the result.

SEETHING

Karzai has taken to seething to reporters about his perceived mistreatment at the hands of Obama officials.

Speaking about himself in the third person to a French journalist last week, he said: "The Americans attack Karzai in an underhand fashion because they want him to be more tractable. They are wrong. It is in their interest ... that Afghanistan's people respect their president."

U.S. policy makers have long understood that this autumn would be crunch time for Afghanistan policy. With thousands of new troops pushing into Taliban-held areas, the summer just past was always going to be the deadliest period of the war.

It lived up to that billing: July was the deadliest month of the war for U.S. troops until August surpassed it.

Polls show the U.S. public losing patience. Democratic members of Congress are grumbling.

In the past week, influential newspaper columnists George Will and Nicholas Kristof -- representing the right and left of the U.S. political spectrum -- came out in favour of an early exit. Another, Thomas Friedman, expressed deep reservations.

If public support for the war has waned in America, it has cratered in Europe, where Washington still relies on allies for political support and 40,000 troops.

Britain and Germany called on Sunday for a conference later this year that would set new targets for Afghanistan to take over its own security and let Western troops withdraw.

"As the Afghans take on more responsibility for their security, then the international engagement can be reduced," German Chancellor Angela Merkel said at a news conference flanked by British Prime Minister Gordon Brown.

In a joint letter with French President Nicolas Sarkozy to the head of the United Nations, they called for "new benchmarks and timelines" for handing over responsibilities to Afghans. Brown offered on Wednesday to host the conference.

While his allies appear to be looking for an exit, Obama is contemplating sending more troops. Karzai has not made that decision any easier. (For more on Afghanistan, click [ID:nAFPAK]) (For more Reuters coverage of Afghanistan and Pakistan, see:

here

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