Word of the review came as Sunni Arab leaders repeated accusations of fraud after initial reports from the provinces suggested the constitution had passed.

Among the Sunni claims are that police took ballot boxes from heavily “no” districts, and that some “yes” areas had more votes than registered voters.

The Electoral Commission made no mention of fraud and an official with knowledge of the election process warned that it was too early to say whether the unusual numbers were incorrect or if they would affect the outcome.

But questions about the numbers raised tensions over Saturday’s referendum, which has already sharply divided Iraqis.

Most of the Shiite majority and the Kurds – the coalition which controls the government – support the charter, while most Sunni Arabs sharply opposed a document they fear will tear Iraq to pieces and leave them weak and out of power.

Irregularities in Shiite and Kurdish areas, expected to vote strongly “yes”, may not affect the outcome.

The main electoral battlegrounds were provinces with mixed populations, two of which went strongly “yes”. There were conflicting reports whether those two provinces were among those with questionable figures.

In the vote count, a sandstorm also became a factor, preventing many tallies from being flown from the provinces to Baghdad, where they are to be compiled and checked. The Electoral Commission said it needed “a few more days” to produce final results, citing the need for the audit.

At Baghdad’s counting centre, election workers cut open plastic bags of tally sheets sent from stations in the capital and its surroundings – the only ones to have arrived so far.

Nearby, more workers, dressed in white T-shirts and caps bearing the election commission’s slogan, sat behind computer screens punching in the numbers.

Election officials in many provinces have released their initial counts, indicating that Sunni attempts to defeat the charter failed.

But the commission found that the number of “yes” votes in most provinces appeared “unusually high” and would be audited, with random samples taken from ballot boxes to test them, said the commission’s head, Adil al-Lami.

The high numbers were seen among the nine Shiite provinces of the south and the three Kurdish ones in the north, al-Lami told The Associated Press.

Those provinces reported to AP “yes” votes above 90%, with some as high as 97 and 98%.

Two provinces that are crucial to the results – Ninevah and Diyala, which have mixed Sunni, Shiite and Kurd populations – were not among those that appeared unusual, al-Lami said. He said their results “were reasonable and balanced according to the nature of the population in those areas”.

But the official with knowledge of the counting process said the unexpected results were not isolated to the Shiite and Kurdish provinces and were “all around the country”.

Sunni opponents needed to win over either Diyala or Ninevah to veto the constitution. Sunnis had to get a two-thirds “no” vote in any three of Iraq’s 18 provinces to defeat the charter, and they appeared to have got it in western Anbar and central Salahuddin, both heavily Sunni.

Ninevah and Diyala are each believed to have a slight Sunni Arab majority. But results reported by provincial electoral officials showed startlingly powerful “yes” votes of up to 70% in each.

Allegations of fraud in those areas could throw into question the final outcome. But questions of whether the reported strong “yes” vote there is unusual are complicated by the fact that Iraq has not had a proper census in 15 years, meaning the sectarian balance is not firmly known.

A prominent Sunni Arab politician, Saleh al-Mutlaq, claimed Diyala in particular had seen vote rigging. He said he was told by the manager of a polling station in a Kurdish district of Diyala that 39,000 votes were cast although only 36,000 voters were registered there.

Al-Mutlaq said soldiers broke into a polling station in a Sunni district of the Diyala city of Baqouba and took ballot boxes heavy with “no” votes and that later results showed a “yes” majority. His claims could not be independently verified.

“Bottom line, we can say that the whole operation witnessed interference from government forces,” he said.

Al-Mutlaq and Sunni Arab parliament member Meshaan al-Jubouri said polling officials in Ninevah had informed them that the provincial capital, Mosul, voted predominantly “no” – as high as 80% – while the Electoral Commission reported a 50-50 split.

But Ninevah’s deputy governor, Khesro Goran, a Kurd, dismissed the claims. “These declarations are excuses to justify the loss, and we did not receive any complaint from the (Electoral Commission) about such fears.

"Besides, the whole operation was under the supervision of the United Nations ... so no fraud occurred,” he said.