The government on Sunday urged Iraqis to vote in next week's constitutional referendum, condemning insurgent groups for demanding a boycott and for killing scores of civilians in an effort to wreck the ballot.

"These insurgents are like rats spreading plague among the people," said Laith Kuba, the main Iraqi government spokesman. "Rats are very small, but the disease they spread is horrible. Iraq should bet rid of these dirty rats," he said at a news conference in Baghdad.

Kuba said a successful boycott would weaken Iraq and delay its efforts to rebuild, and that "the killing of innocent civilians is now the nation's No. 1 challenge."

Sunni-led insurgent groups are trying to reduce turnout in the referendum with a wave of attacks, and at least 311 people have died in the last two weeks in homicide bombings, roadside bombs, drive-by shootings and the assassination of people who had been kidnapped.

That death toll includes two U.S. soldiers who were killed Saturday in fighting in western Iraq, which brought to eight the number of American fatalities in a series of offensives the military has launched to put down militants before the Oct. 15 vote, especially in Iraq's western province of Anbar, the heartland of the insurgency.

Kuba also expressed concern that more than 500 corpses have been found in Iraq since its interim government was formed April 28. He said Shiites and Sunnis have been both been the victims of such attacks, and he blamed them on insurgents who have infiltrated, or disguised themselves as, Iraqi security forces, and undisciplined Iraqi policemen or soldiers.

The Iraqi and U.S. governments are working hard to get the constitution approved, but Kuba said even minority Sunnis who oppose the draft document should benefit from Iraq's democratic reforms by going to polling stations and voting "no."

Minority Sunni Arabs are gearing up their campaign to defeat the measure at the polls. The referendum has divided Iraqis, with leaders of the Shiite Muslim majority and Kurds supporting the constitution and Sunni Arabs opposing it, saying it will fragment Iraq.

Sunnis can defeat the charter if they garner a two-thirds "no" vote in any three of Iraq's 18 provinces. The Sunnis have a majority in four provinces.

Iraq on Saturday announced a curfew, weapons ban, border closings and other security measures to clamp down ahead of the referendum and to prevent insurgent attacks.

Iraqi officials are distributing 5 million copies of the constitution to the public ahead of the vote, often leaving them at small shops in cities and towns that act as ration centers where most Iraqis get government-subsidized food.

But some shops were refusing to participate, fearing attacks by insurgents.

In Baqouba, a city 60 kilometers (35 miles) northeast of Baghdad, police and soldiers began distributing tens of thousands of copies of the constitution at schools and bus stations Sunday, said official Hafiz Abdel-Aziz. "We decided not to distribute them through food ration agents for security reasons," he said in an interview with The Associated Press.

The same fears were prompting officials to distribute tens of thousands of copies of the document at schools, mosques and government buildings in the northern province of Kirkuk.

On Sunday, three Iraqi contractors and government bodyguard were killed by insurgent gunmen in three separate attacks in the cities of Baghdad, Beiji and Mosul, police said.

In southern city of Basra, a homicide car bomb killed two people — and woman and a child — and wounded three Iraqis, police said.

It exploded early Sunday morning outside a three-story apartment building used by the Iranian-backed Badr Brigade (search), a Shiite militia linked to one of the main parties in the Iraqi government. Iraqis who were present but escaped injury included former Basra Gov. Hassan al-Rashid, a senior local leader of the brigade, said police Capt. Mushtak Kadim.

The Badr Brigade is the military wing of the Supreme Council of Islamic Revolution in Iraq (search), or SCIRI, the largest Shiite party in the Iraqi government.

It was not immediately known who had carried out the attack in Basra (search), a mostly Shiite city where the majority of Britain's 8,500 forces are based.

Despite earlier claims that British soldiers had created much better security in southern Iraq than other areas of the country beset by Sunni-led insurgent groups, Shiite militias appear to have been growing in power in the mostly Shiite region, infiltrating police forces and local political organizations, and allegedly attacking British and U.S. forces.

The Guardian, a British newspaper, quoted Basra's police chief, Hassan al-Sade, in May as saying that the militias had become the "real power" in Basra and that he trusted only 25 percent of his own police force.

This summer, Al Qaeda in Iraq, the militant group led by Jordanian-born Abu Musab al-Zarqawi, threatened in an audiotape to unveil a new unit to eradicate the Badr Brigade.

But fighting also has occurred in this region between the Badr Brigade and other Shiite militias, including the al-Mahdhi (search) militia, which is associated with the firebrand cleric Muqtada al-Sadr. Another Shiite power is the Fadila Party (search), which won tenuous control of the provincial government this year from SCIRI after allying with smaller parties.

On Saturday, a delegation from the Arab League (search) arrived in Iraq to lay the groundwork for an Iraqi "reconciliation conference" it hopes to hold after the referendum vote.

It was the first time the pan-Arab organization has tried to take a direct role in Iraq since the 2003 U.S.-led invasion.

"The situation is so tense there is a threat looming in the air about civil war that could erupt at any moment, although some people would say that it is already there," Arab League Secretary-General Amr Moussa said in a BBC interview on Saturday.

But the organization has gotten a cold reception from some Shiite leaders in the government, resentful over perceived Arab League inaction in response to Saddam Hussein's (search) regime and what they see as the predominantly Sunni league's bias in favor of Iraq's Sunni minority.

During his news conference Sunday, Kuba urged the League to improve its relations with Iraq by following the example of the United Nations (search) and opening an office in Baghdad.

He said the League must realize that many Iraqis resent anyone who supported Saddam's regime as it killed hundreds of thousands of innocent citizens. But, Kuba said, "It is now in our interest to establish good relations with the Arab League."