Figures provided by several governorates required "re-examination, comparison and verification because they are relatively high compared with international averages for elections" of this kind, a statement by the Independent Electoral Commission of Iraq said on Monday.
The commission's statement came as Sunni Arab lawmaker Mishaan al-Jubouri claimed fraud had occurred in the vote - including instances of voting in hotly contested regions by pro-constitution Shia Muslims from other areas - repeating earlier claims of election-fixing made by other Sunni Muslim officials over the weekend.
"Despite its intention to announce global preliminary results as soon as possible, it needs several more days to complete this difficult and complex operation after finding that figures from most provinces were too high," said the commission's statement in an apparent reference to turnout levels.
"Statements coming from most governorates indicating such high numbers that require us to recheck, compare and audit them, as they are unusually high according to the international standards," the commission said.
The commission said it would take random samples from ballot boxes from areas reporting very high percentages or too low percentages in the results.
It did not specify which provinces the unusual reports were coming from, or say whether the unusual numbers could affect the outcome.
Two Sunni-dominated provinces
may have rejected the charter
The electoral body added that it would "choose urns at random and examine them again, in particular where the figures were too low or too high compared with the general average.
"The commission will only announce results when they have been verified."
A source close to the commission said: "The problems are not in Sunni Arab zones."
Asked why most observers expect Mosul to be crucial to the referendum's outcome, al-Jiburi said: "It is true that Mosul is a decisive factor given that two governorates, Salaheddin and Anbar, have rejected the constitution - there is no room for controversy over their results. As for Mosul, if there is no fraud in the counting of the city's votes, the final outcome will be a no for the constitution."
While sources close to the electoral commission were quoted as saying that 55% of voters in Nineveh governorate had said no to the constitution, the secretary-general for the Iraqi Independent Front, Abd al-Razaq al-Jiburi, said more than 75% of voters had voted no.
Al-Jiburi told Aljazeera: "I have been informed by an employee of the electoral high commission in Mosul that the voting for the constitution has been no.
"Figures from many polling stations suggest the percentage of those who have voted no to the constitution ranges from 75% to 80%.
"Observers and employees at the electoral commission provided these figures," al-Jiburi added.
Shia voting trend
Earlier on Monday, a commission spokesman said six majority Shia Muslim provinces in southern Iraq had voted by more than 90% in favour of the constitution, which aims to lay a foundation for the country following the ousting of Saddam Hussein's government.
"Statements coming from most governorates indicating such high numbers that require us to recheck, compare and audit them, as they are unusually high according to the international standards"
Independent Electoral Commission of Iraq
The six provinces were Basra, Dhi Qar, Karbala, Misan, Najaf and Wasit, spokesman Farid Ayyar said.
Iraq's draft constitution requires a simple majority to pass ,but could be rejected if two-thirds of voters in three or more governorates vote no.
Two Sunni Muslim-dominated provinces appeared to have rejected the text, by 80% in Salaheddin and by 54% in Diyala, so the results from the two other Sunni-dominated provinces in western and northern Iraq were crucial.
The tally from western Anbar and northern Nineveh, which includes the mixed city of Mosul, were not yet known, Ayyar said.