In preparation for national elections this week, the Iraqi government says the country's borders will be closed and curfew extended for at least four days, beginning Tuesday.

The security measures the Iraqi government outlined Sunday are similar to measures that were used to protect voters in January and again, in October, when Iraqis went to the polls to vote on a new constitution.

All international border crossings and airports will be closed and travel between provinces banned from Tuesday to Saturday. Nighttime curfew will also begin two hours earlier, at 10 o'clock, and last until six a.m.

The main balloting will take place on Thursday, but the government says Iraqi security forces, hospitals and prisons for inmates who have not been convicted of any offences can begin voting Monday.

Iraqis living outside the country can vote starting Tuesday at special centers in 15 countries.

Last Friday, clerics at Sunni mosques across Iraq urged followers to participate in elections. Some called voting a religious duty, raising hopes that Thursday's balloting to elect a new, four-year government will be relatively peaceful.

In January, Sunni Arabs, who make up the biggest bulk of the insurgency, largely boycotted Iraq's first post-Saddam elections.

The boycott grew partly out of anger at being sidelined by rival Shi'ite Muslims and Kurds after the fall of Saddam Hussein and partly out of fear amid Sunni extremist threats to disrupt the vote.

Extremists are again threatening to kill anyone who participates in Iraq's political process. But in recent months, the vast majority of Sunni Arabs have made clear that there will be no repeat of the January boycott, which left the religious minority virtually powerless in government.

The United States has welcomed greater Sunni political participation as a way to create a more broad-based constitutional government, which can diffuse escalating sectarian and ethnic tensions in the country, calm the insurgency, and allow U.S. forces to withdraw.

The U.S. Ambassador to Iraq, Zalmay Kalilzhad, says it crucial for Iraqis to choose good leaders who can heal, not further divide, the fragile nation.

"Every Iraqi should educate themselves about which candidate or list has correctly identified the challenges that Iraq faces; which candidate or list has the right plans to deal with the country's problems, and which candidate or list is more likely to deliver on the promises that they are making," he said. "Also, they should discuss what the individual candidates or lists have done for the people of Iraq already. Have they delivered on past promises?"

There is also much riding on these elections for Iraq's Shi'ite majority, who say they are determined to retain the upper hand after decades of oppression under Sunni-Arab rule.

Iraqi Kurds are no less determined to preserve their autonomous status in the north and to incorporate the disputed oil-rich city of Kirkuk into their region. Iraqi Arabs and Turkmen, who also claim Kirkuk as their own, strongly reject Kurdish claims over the city.

On Sunday, Iraq's electoral commission said it was investigating possible violations in the voter registration in Kirkuk. The commission said it found a five-fold increase in the number of new voters there in the past two days. Election officials did not say which group - the Kurds, the Arabs or Turkmen - was responsible for the irregularity.