Washington has called for a "full and thorough review of all reasonable allegations of irregularities" in Afghanistan's presidential election after official preliminary results were announced on July 7.
U.S. State Department spokeswoman Jen Psaki said review measures already agreed by both candidates could affect more than 3 million of the 8 million ballots that were counted.
Afghanistan's Independent Election Commission said that preliminary results from the June 14 second-round ballot show Ashraf Ghani with 56.44 percent of the vote compared to 43.56 percent for Abdullah Abdullah.
Commission chairman Ahmad Yousuf Nouristani stressed that the results could change after taking into consideration allegations of fraud or a review of votes that could later be invalidated.
Ghani, a former finance minister, and Abdullah, a former foreign minister, have accused each other of trying to manipulate the results.
Speaking to RFE/RL's Radio Free Afghanistan shortly after the preliminary results were announced, Fazel Rahman Orya, a spokesman for Abdullah's electoral team, said the Independent Election Commission had "no legitimacy" and that he considered the results "illegal" and "illegitimate."
Abdullah has previously said he will not accept any results until an investigation into all allegations of fraud is completed.
Noman Shinwary, a spokesman for Ghani's campaign team, urged Abdullah's team to join in the process of checking complaints filed by both sides.
Nouristani admitted there had been "technical mistakes" and "shortcomings" in the election process, including some incidents of fraud and violations. But he also asked both candidates to "calm down."
Candidates have 48 hours to file formal complaints.
A July 22 deadline has been set to announce the final results.
Representatives of the two rival candidates met ahead of the release of the preliminary results to discuss how to defuse tensions over the disputed vote count.
The hotly contested outcome of the election has sparked concerns about a wider split along ethnic lines in Afghanistan.
Ghani attracts support from Pashtuns in the south and east, while Abdullah's loyalists are Tajiks in the north -- echoing the ethnic divisions of the civil war in the 1990s.
The international community had pressed for a smooth transition of power in Afghanistan ahead of the planned withdrawal of international forces by the end of 2014.
With reporting by Reuters, AP, AFP, and dpa