Independent Afghan election monitors say allegations of major fraud have more than doubled in the past two days and that investigators are now looking into more than 550 reported incidents.
Investigations into the latest fraud allegations, reported by the Electoral Complaints Commission, could further delay a vote counting process that has been much slower than officials predicted.
The latest official results from about one-third of polling stations show incumbent President Hamid Karzai leading with 46 percent. His closest challenger, Abdullah Abdullah, has 31 percent.
A candidate needs to get more than 50 percent of the vote to avoid a two-man runoff.
Even before the August 20 poll, presidential candidates said fraudulent voter registrations, secret political deals and low voter turnout could imperil the election's legitimacy.
Mr. Karzai has been criticized for recruiting former warlords to help win votes. Abdul Rashid Dostum, widely reported to be partly responsible for the alleged massacre of some 2,000 Taliban prisoners, campaigned in Afghanistan for the president last week.
The U.S. embassy in Kabul released a statement Sunday saying officials maintain serious concerns about the warlord. U.S. officials said they have not been approached by Dostum and have no plans to meet with him.
Some information for this report was provided by AP and Reuters.