U.S. officials on Tuesday warned Afghan presidential candidate Abdullah Abdullah against endorsing violence or extra-constitutional measures and cautioned that a protracted stalemate over the fraud-marred June 14 election could have dire consequences for Afghanistan.

The embattled politician defiantly mobilized thousands of supporters in the heart of the capital Tuesday as he declared himself the winner and vowed to challenge preliminary results that suggest he lost by a significant margin.

“You are the victorious; you have won the vote — there is no question,” Abdullah shouted to a cheering crowd at a spacious conference hall in western Kabul. “We would rather be torn into pieces than accept this fraud.”

The former foreign minister alleges that election officials rigged the vote in favor of his opponent, former finance minister Ashraf Ghani, in the runoff vote.

U.S. officials who had hoped the election would lead to a smooth transfer of power by the end of the summer have grown increasingly concerned this week as the crisis has deepened, jeopardizing the Obama administration’s plan to keep a small international military contingent in Afghanistan after 2014.

Afghan presidential candidate Abdullah Abdullah has vowed to challenge preliminary election results amid accusations of fraud. Secretary of State John F. Kerry will arrive in Kabul on Friday to help mediate the crisis. (Massoud Hossaini/AP)

Secretary of State John F. Kerry, who is in Beijing, is planning to stop in Kabul to meet with both candidates on his way back home Friday.

President Obama called Abdullah on Monday to warn him against actions that could lead to “violence”or “extra-constitutional measures,” White House spokesman Josh Earnest told reporters Tuesday.

“We’ve been clear that any such move would cost Afghanistan the financial and security assistance of the United States of America,” Earnest said.

There were concerns in Kabul that Abdullah and his team would use Tuesday’s rally to declare a parallel government, which would have aggravated Afghanistan’s political crisis and raised the risk of bloodshed. But Abdullah stopped short of announcing his own cabinet, drawing jeers from the crowd, which urged him to declare himself president.

In a statement issued Monday night, Kerry suggested that the United States is concerned that Afghanistan’s fledgling security forces­ could get dragged into the political fray.

“The apolitical role of the security forces­ must be respected by all parties,” the statement said. “We call on all Afghan leaders to maintain calm in order to preserve the gains of the last decade and maintain the trust of the Afghan people.”

The State Department’s special representative for Afghanistan and Pakistan, James F. Dobbins, said Tuesday that Kerry hopes to “get the process back on track” by encouraging both candidates to endorse a thorough review of ballots that could weed out those cast fraudulently. The secretary also hopes to “encourage a dialogue between the two candidates,” Dobbins told TOLO, an Afghan news network.

Dobbins ruled out the possibility of a new election, saying it would “take too long to organize,” but he acknowledged that the U.S. government has concluded that “there was very substantial fraud.”

The envoy warned that the consequences of a sustained standoff could be severe because Afghanistan is heavily reliant on foreign aid.

“The consequences of a division in Afghanistan and a cessation of international support would mean that the police would stop getting paid, the military would stop getting paid, clinics would close, schools would close, electricity would begin to go off,” Dobbins said. “Very soon the great progress that Afghanistan has made over the last 13 years would begin to roll back.”

On Monday, Afghanistan’s Independent Election Commission announced the initial results of last month’s runoff, putting Ghani, a former World Bank executive, in the lead with 56 percent. The tally showed Abdullah with 44 percent of the vote.

The Abdullah camp had alleged weeks earlier that Ghani supporters, working in collusion with electoral officials, had stuffed ballot boxes in a systemic way.

Election authorities agreed to audit 1,930 suspect polling stations, but it seemed unlikely that a review of those votes could overturn the results. Representatives from both campaigns had been engaged in eleventh-hour negotiations to expand the scope of the audit to encompass as many as 7,000 polling sites where the voting patterns were suspicious.

Abdullah’s campaign had urged the commission to refrain from announcing any figures until fraudulent votes were separated from the count.

Kenneth Katzman, an expert on Afghanistan at the Congressional Research Service, said he was pessimistic about the prospect of a prompt resolution, given the apparent scope of the fraud and the candidates’ entrenched positions.

“This is going to be very difficult to untangle,” said Katzman, who has written several reports on U.S. policy on Afghanistan for members of Congress. “There is no easy mechanism.”

Katzman said the fight could easily turn violent, has the potential to embolden the Taliban and could leave President Hamid Karzai in office well beyond the time frame of late summer that Western officials saw as a best-case scenario. That would be problematic for the Obama administration because Karzai has refused to sign a security agreement with Washington that would pave the way for a post-2014 foreign military footprint in the country, which donor nations have established as a precondition for continued international support. Abdullah and Ghani had vowed they would sign the deal soon after taking office.

U.S. officials were hopeful that a new president would have been sworn in before a key meeting of NATO officials scheduled for early September in Wales.

“I think all along we have wanted this process to play out in time for the NATO summit because international commitments need to be made,” said a senior U.S. defense official who was not authorized to discuss the matter on the record.

Londoño reported from Washington. Scott Wilson and Katie Zezima in Washington and Sayed Salahuddin and Mohammad Sharif in Afghanistan contributed to this report.