The Iraqi National Alliance (INA), a largely Shi'ite group that is challenging Maliki's bid to win a second term, told a news conference that according to their informal tallies the prime minister was ahead in at least 10 of Iraq's 18 provinces.

A decisive victory by any political coalition is unlikely in an election Iraqis hope will bring a measure of stability after years of sectarian warfare as US troops prepare to pull out. Negotiations to form a new government could take months.

Iraq has been on edge since Sunday's vote, during which 39 people were killed in attacks, but electoral officials say they require at least another day to reach the 30 percent threshold needed to declare preliminary results.

Final results may require several more weeks.