Baghdad, Iraq ? Iraq announced a curfew, weapons ban, border closings and other security measures Saturday to clamp down ahead of next weekend’s key constitutional referendum and prevent insurgent attacks. Sunni Arabs geared up their campaign to defeat the measure at the polls.

Two U.S. soldiers were killed in fighting in western Iraq, bringing to eight the number of American casualties in a series of offensives the military has launched to put down militants before the Oct. 15 vote.

In Baghdad, a suicide attacker detonated a car full of explosives at two police vehicles forming a checkpoint, killing at least five policemen and wounding 20 people, including six civilians.

It was the sort of attack Iraqi security forces are hoping to avert with a ban on using vehicles on voting day – a step taken during parliamentary elections in January. Sunni-led insurgents have vowed to wreck the referendum with a wave of attacks.

“We will protect those who say yes and those who say no,” Interior Minister Bayan Jabr said in Baghdad. “We have countermeasures against all terrorist actions, and you will see tens of thousands of Iraqi security forces deployed in Baghdad and the provinces.”

On Thursday – two days ahead of the vote – a nationwide nighttime curfew will begin and nobody will be able to carry weapons in public, even if they are licensed, Jabr said. On Friday evening, police will bar travel between provinces. International borders, airports and ports also will be closed, but Jabr did not say when that step would begin.

He acknowledged problems with security in Iraq’s western province of Anbar, the heartland of the insurgency. In the provincial capital of Ramadi, only 1,000 of the city’s 6,500-member police force were willing to come to work, Jabr said. He said help from powerful local tribes was needed to protect polling stations and the Iraqi military would have to be responsible for security.

‘Reconciliation conference’

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.

A delegation from the Arab League arrived Saturday in Iraq to lay the groundwork for an Iraqi “reconciliation conference” it hopes to hold after the 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 warned in an interview with British Broadcasting Corp. radio 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 regime and what they see as the predominantly Sunni league’s bias in favor of Iraq’s Sunni minority.

Operation River Gate

In western Iraq, U.S. troops continued their four-day-old Operation River Gate, a sweep of several towns along the Euphrates River valley, one of two offensives launched last week in Anbar province against insurgents.

The two U.S. soldiers were killed Friday by small arms fire near the town of Haqlaniyah during the operation.

Six other U.S. service members have been killed in past days in River Gate and the second operation, Iron Fist, which ended Thursday night further upriver near the Syrian border. At least 1,952 U.S. service members have died since the Iraq war started in March 2003, according to an Associated Press count.

The military had said the towns were under insurgents’ control, but most of the fighters appear to have fled or gone into hiding before the sweeps – leaving Marines and soldiers to deal with booby-trapped streets.

In Haditha, near Haqlaniyah, Iraqi commanders accompanied by U.S. Marines walked through neighborhoods, telling residents that security forces would remain in the city and handing out fliers urging residents to call in tips about the insurgency.

Other violence

In the latest violence, insurgents killed a local council member in the northern city of Kirkuk in a drive-by shooting. Gunmen also killed a police captain in Baghdad and another in Samarra.

Iraqi soldiers opened fire on a civilian car that failed to stop at a checkpoint in Baghdad, killing the driver and wounding five other occupants, including a child. Four bodies were found dumped in parts of the capital.