* Intense diplomacy before ECC announcement

* Second round runoff a possibility, officials say

* Karzai talks to Clinton, U.N., British leaders (Adds John Kerry comments)

By Maria Golovnina

KABUL, Oct 17 (Reuters) - Senior foreign officials pressed President Hamid Karzai on Saturday to resolve a disputed election that threatens to undermine the Western effort to stabilise Afghanistan amid a growing Taliban insurgency.

Allegations of fraud in the Aug. 20 presidential election have left Afghanistan in a state of political uncertainty as U.S. President Barack Obama is deciding on sending more troops to fight the Taliban.

U.S. Senate Foreign Relations Committee Chairman John Kerry said on Saturday that Washington should not proceed with a new Afghan strategy involving more troops without a clear partner in Kabul.

Kerry, who was defeated by George W. Bush in the 2004 presidential campaign, was among several high-level visitors to Afghanistan before the U.N.-backed Electoral Complaints Commission's expected announcement this weekend on whether there will need to be a runoff between Karzai and former Foreign Minister Abdullah Abdullah.

"Look, it would be entirely irresponsible for the president of the United States to commit more troops to this country, when we don't even have an election finished and know who the president is and what kind of government we're working in," Kerry said in an interview with CNN's "State of the Union" from Kabul. [nN17307546]

The election is a vital element in Western plans to stabilise Afghanistan and deny sanctuary there to militants believed to have used it as a base for actions across the globe, including the Sept. 11, 2001, attacks on the United States.

U.S.-led troops ousted the Taliban government after the Sept. 11 attacks for offering safe haven to al Qaeda, but the Islamist movement has regrouped into a formidable insurgency.

More than 100,000 foreign troops are in Afghanistan fighting the Taliban, but growing casualties and doubts about the Karzai government are undermining support for the effort in the United States, Britain and other countries involved.

SECOND ROUND 'ON RADAR SCREEN'

A Western official said a second election round "is definitely on the radar screen right now. This is why there are delays. There are some tense negotiations going on."

The complaints commission has already said it had found "clear and convincing evidence of fraud."

If it finds the fraud insufficient to overturn the result, Karzai can be declared the winner and would move to appoint a new government.

Karzai has made clear he would prefer not to fight a second round and has spoken out against the investigation, making veiled accusations of foreign meddling.

Some officials believe Karzai may resist accepting the commission's findings or seek to overturn them.

A second round carries risks of new violence by Taliban insurgents, weeks more of instability, heightened ethnic tensions during the campaign and a second flurry of controversy over fraud.

"This is very complicated," French Foreign Minister Bernard Kouchner, also visiting Kabul, told reporters. "We want to understand why it is not possible to get a consensus. But you need to work together."

Karzai separately spoke by phone to British Prime Minister Gordon Brown, U.N. Secretary-General Ban Ki-moon, Pakistan President Asif Ali Zardari and U.S. Secretary of State Hillary Clinton, Karzai's office said.

Karzai won 54.6 percent of the vote, according to preliminary figures. More than 250,000 votes would have to be thrown out from his tally for it to fall below 50 percent.

If enough votes were disqualified, the incumbent would face Abdullah in a second round -- barring possible legal steps to invalidate the decision or an Abdullah decision to withdraw.

While accusing Karzai's camp of fraud and calling for a second round, Abdullah has hinted he might be open to some discussions after the first round result is announced.

A runoff would be due within two weeks. (Additional reporting by Golnar Motevalli, Hamid Shalizi and Susan Cornwell; Editing by Angus MacSwan and Peter Cooney)

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