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Afghan presidential candidate Ashraf Ghani Ahmadzai smiles as he listens during a news conference in Kabul July 8, 2014. The United States warned on Tuesday it would withdraw financial and security support from Afghanistan if anyone tried to take power illegally, as supporters of presidential candidate Abdullah Abdullah rallied in Kabul for a parallel government. REUTERS/Mohammad Ismail

U.S. President Barack Obama Tuesday threatened to cut off aid to Afghanistan if "violent or extra-constitutional means" are employed in the wake of the disputed presidential election.

Candidate Abdullah Abdullah told supporters Tuesday he will declare victory, claiming results favoring Ashraf Ghani Ahmadzai were fraudulent, the Associated Press reported. Preliminary results of the June 14 runoff released by the Afghan Independent Election Commission Monday showed Ahmadzai leading Abdullah by 1 million votes.

Obama spoke with both Ghani and Abdullah, calling for calm as violence escalated around Afghanistan, including a suicide bombing in Parwan province that killed at least 16 people.

Obama urged a thorough review of election results to ensure credibility, a White House readout of the conversation said.

"He reiterated that all parties should avoid steps that undermine Afghan national unity and should come together to work toward a resolution that represents the will of the Afghan people and produces a government that can bring Afghanistan together," the readout said. "He also noted that there is no justification for resorting to violent or extra-constitutional means, which would result in the end of U.S. assistance to Afghanistan."

Secretary of State John Kerry is expected to travel to Afghanistan Friday.