BAGHDAD (AFP) -- Iraq's general election expected to take place on January 18 will be vital for national reconciliation, but ensuring it goes ahead on time will be a herculean task, the United Nations said on Wednesday.

The election will be the second national poll since the 2003 American-led ouster of Saddam Hussein, and is seen as crucial to consolidating the war-torn country's fledgling democracy and ensuring a smooth US military exit.

Faraj al-Haidari, head of Iraq's election authority, the Independent High Electoral Commission (IHEC), told reporters in Baghdad the date had been changed from January 21 to January 18 because of a religious ceremony.

"I was informed by them that January 18 would be better than January 21," Haidari said of the presidential council that comprises President Jalal Talabani and the two vice-presidents.

"The decision is theirs and we know from them orally that it will be on January 18," he added, referring to what is now the third date to have been touted for the vote.

Arbaeen, marking 40 days after the Ashura anniversary of the killing of Imam Hussein, grandson of the Prophet Mohammed, forced the date change because pilgrims will be travelling on foot across the country at the time.

The UN secretary general's envoy to Iraq, Ad Melkert, speaking at a news conference with Haidari, said the 10-week timeline for organising a January 18 election would be difficult but was possible.

"I must emphasise that the time is really very, very short," Melkert said.

"This election will play a major role in ensuring that Iraq moves forward with national reconciliation, development and reconstruction."

But he added: "There are less than 10 weeks available to organise these elections, which is truly a herculean task. IHEC is under immense pressure to deliver a credible election in a very short period of time."

After weeks of wrangling, MPs on Sunday finally passed the electoral law that will govern the vote, which is set to be fought in circumstances vastly different from the previous general election in December 2005.

Sectarian strife between the country's Shiite and Sunni communities was then rising. At its peak in 2006, this led to an average of 63 people being killed each day, compared with fewer than 10 each day so far this year.

The election was originally scheduled for January 16, but IHEC on Monday said it had been changed to January 21 in the wake of repeated rows among MPs over whether an open or closed voting system would be used.

Working out how the ballot would proceed in the ethnically divided oil province of Kirkuk was also a major factor in causing repeated hold-ups on passing the law.

A compromise saw MPs decide that the election result will be provisional in Kirkuk and other provinces where there is disagreement over electoral rolls because of a high recent increase in respective Kurd and Arab populations.

US Ambassador Christopher Hill said on Sunday the planned US troop withdrawal would go ahead as scheduled now that the electoral law is in place.

There are 117,000 American soldiers in Iraq. All combat troops are due to leave the country by August 2010 ahead of a complete pullout by the end of 2011.

Under the open voting system, electors will choose either a single named candidate -- which favours high-profile politicians -- or a party, and will have much more choice on their ballot papers than last time.

Two Shiite coalitions, including the State of Law grouping headed by Prime Minister Nuri al-Maliki, two Sunni blocs, two secular lists and three Kurdish groups are the main contenders.

However, a total of 296 parties are set to campaign, compared with just 12 party lists that won seats in 2005.

The law guarantees that 25 percent of MPs will be women and allocates at least eight seats for minorities, including five for Christians.

By Mehdi Lebouachera