(For more stories on Afghanistan, click [ID:nAFPAK])

* Run-off cancellation a double-edged sword for Obama

* Obama says Karzai must deliver on promises

* White House presses Karzai to curb corruption

* Decision on sending more troops still weeks away

(Adds Obama comments, analyst, anti-corruption efforts)

By Ross Colvin

WASHINGTON, Nov 2 (Reuters) - U.S. President Barack Obama signaled on Monday that Washington's support for Afghan leader Hamid Karzai would come with more strings attached, including a demand he get much tougher on rampant corruption.

Obama called Karzai to congratulate him on winning a second five-year term after Afghan election officials scrapped a Nov. 7 run-off vote. Karzai's only challenger, former foreign minister Abdullah Abdullah, withdrew from the election on Sunday, citing doubts about the fairness of the process.

"I emphasized that this has to be a point in time in which we write a new chapter based on improved governance, a much more serious effort to eradicate corruption (and) joint efforts to accelerate the training of Afghan security forces," Obama said in the White House Oval Office.

Karzai's re-election capped weeks of political uncertainty that has complicated Obama's efforts to revamp U.S. strategy in Afghanistan and decide on a request by his top commander there for up to 40,000 more troops to tackle deteriorating security.

But analysts said Obama now faced an uphill battle in convincing a skeptical American public that Karzai deserved continued U.S. support, after a fraud-riddled election that saw millions of ballots favoring the Afghan leader thrown out.

"It is going to be harder to sell domestically. Arguably the most consequential effect (of the election) has been here," said Stephen Biddle, who helped advise U.S. Afghan commander General Stanley McChrystal in a rethink of U.S. policy there.

"They (the administration) are definitely going to attach more systematic strings to the thousands of things we do in Afghanistan every day. They are going to insist on government reforms, without which the campaign will fail," said Biddle, a military analyst.

White House spokesman Robert Gibbs faced repeated questions at a briefing on how Washington could work with Karzai when his legitimacy had been so widely questioned after the tainted August vote.

"President Karzai has been declared the winner ... so obviously he is the legitimate leader of the country," said Gibbs, without directly addressing questions on whether the administration also saw him as credible.

DEEDS NOT WORDS

In his comments later, Obama said Karzai had assured him that he understood the importance of improving governance and stamping out corruption, "but as I indicated to him, the proof is not going to be in words, it is going to be in deeds."

Obama administration officials want Karzai to sign onto measures to combat corruption. One option under discussion is creation of an anti-corruption commission with the authority to go after top officials.

Washington is also ready to offer practical assistance to the next Afghan government, such as sending more civilian advisers to ministries to help improve services to wide swathes of the country, officials said.

Gibbs said Obama would announce his new strategy "in the next few weeks." The timing had never been dependent on the outcome of the election, he said, although administration officials had previously suggested it was a factor.

A senior administration official told Reuters at the weekend that Obama was not likely to announce his new strategy before he embarked on a 10-day trip to Asia on Nov. 11.

The cancellation of the run-off is both a concern and a blessing for Obama.

After U.S. troops suffered their bloodiest month in the eight-year-old war in October, his administration would have been reluctant to commit troops to police an election in which there was only one candidate and which the Taliban had vowed to attack.

But it now has to work with a severely weakened leader whose government is viewed by many Afghans as both corrupt and inept. Relations between Karzai and Washington also soured after the fraud-tainted August election and U.S. officials had to pressure the Afghan president to stand in the run-off.

With the run-off now canceled, Obama is likely to come under more pressure to make a decision soon on his strategy, although the White House has said such an important policy shift requires careful consideration.

Republicans have been particularly critical of Obama, saying he is being overcautious. The House of Representatives Republican leader, John Boehner, said the scrapping of the run-off had removed the "pretext for delaying the decision on giving General McChrystal the resources he needs to achieve our goals in Afghanistan."

There are some 67,000 U.S. troops in the country. (Writing by Ross Colvin; Additional reporting by Adam Entous, Caren Bohan and Steve Holland; Editing by Eric Walsh)