Iraqi Prime Minister Nouri al-Maliki's political party says its investigation into the March 7 parliamentary election has thrown into question about 750,000 votes, enough to change the results of the nationwide poll.
The State of Law alliance trailed by just two seats behind former prime minister Ayad Allawi's party in the vote, which produced no clear winner because neither side got enough seats to govern alone.
Al-Maliki has demanded an official recount as both sides struggle to form a ruling coalition.
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"We estimate the volume of vote manipulation is up to 750,000 votes," Hajim al-Hasani, the spokesman of the State of Law Alliance, told reporters in Baghdad. "This number could change the election results."
More than 11.5 million votes were cast in the election, which international observers said was fair and transparent.
An appeals committee is reviewing the complaints, but there is no timetable for a decision.
Al-Hasani said State of Law examined ballots from five of Iraq's 18 provinces - Basra, Baghdad, Qadissiya, Anbar and Ninevah - and irregularities include unsigned ballots and discrepancies in signatures.
He said his party estimates they were cheated out of up to 15 seats.
Sunday's announcement will likely worsen the political deadlock, which many fear is fuelling a recent wave of deadly violence in an around Baghdad.
The most brutal attacks in the past two weeks include a triple suicide bombing outside foreign embassies in the capital, the execution-style slaying of Sunni villagers and a slew of explosions that ripped through residential areas. More than 120 people died in the attacks.
On Sunday, police said three schoolchildren were killed in a roadside bombing north of Baghdad.
Al-Maliki, who has led a government dominated by religious Shi'ites for the past four years, is facing a serious challenge from Allawi, a secular Shi'ite who says he is looking to transcend the country's ethnic and sectarian divides.
Allawi's list drew on heavy Sunni support to come out ahead, with 91 seats to al-Maliki's 89.
Allawi's Iraqiya party on Sunday called on Iraq's judicial authorities to remain independent and uninfluenced by political pressure.
"We will be committed to the election results due to be announced by judicial authorities if the appeal is carried out with professionalism and objectivity," he said.
Secretary of State Hillary Rodham Clinton said the recent violence in Iraq has not derailed plans for the US military to draw down its forces this year. The US plans to reduce troop levels from more than 90,000 to 50,000 by August 31, when it will end combat operations.
As part of an agreement with Iraq, the US will withdraw all forces by the end of 2011.
"Our withdrawal is on schedule," Clinton told CBS television's Face the Nation program on Sunday.
