With its jumble of ethnic groups, this northern city is one of the pressure points where the new Iraq could blow apart. And if Kirkuk didn't have enough problems already, it now has to worry about what the U.S. military calls "VBEDs."

The acronym stands for "vehicle-borne explosive devices," or, in common parlance, suicide car bombs. They're the latest tactical innovation used by the insurgents in Iraq, and they're making the country's streets and highways even more dangerous for the U.S.-led coalition and its Iraqi allies.

The menace of these mobile bombs was clear here last Friday, as Maj. Jim Hanson was leading a convoy of armored Humvees through the center of Kirkuk. The convoy had just visited a local Iraqi security headquarters when Hanson received an intelligence report over the radio that a roving car bomber might be in the area, driving a black BMW and looking for a target of opportunity.

Suddenly, every black sedan cruising down the road looked like a potential killer. Hanson coolly directed the convoy safely back to a U.S. military base here, and the reported BMW bomber -- if he existed at all -- never attacked.

The car bomb report in Kirkuk is a small example of the insecurity that plagues Iraq today. In Baghdad, these four-wheeled rovers have become a potent assassination weapon: One killed Izzedin Salim, acting president of the Iraqi Governing Council, in May, and another almost killed Iraqi Justice Minister Malik Douhan Hasan on Saturday. On Monday a bomber driving a fuel truck blew himself up at an Iraqi police station, killing at least nine people and wounding more than 60.

The insurgents have turned to these mobile bombs just as the U.S.-led coalition is finding technology that can remotely disarm the fixed "improvised explosive devices," or roadside bombs, that have killed so many Americans and Iraqis. The insurgents' new tactic is to send car bombers out cruising for high-value U.S. or Iraqi targets.

The aim is to sow fear, and it's working. The sense of vulnerability in Iraq these days is pervasive: It threatens U.S. and Iraqi security forces, frightens Iraqi civilians and intimidates foreign workers. In this deadly environment, rebuilding Iraq has proved very difficult.

Kirkuk is a special flash point, where the violence of the insurgency feeds off long-standing ethnic tensions. The city of about 850,000 is roughly 35 percent Kurd, 35 percent Sunni Arab and 26 percent Turkmen, according to U.S. estimates. The Kurds argue that they were dominant here until the former regime began brutally moving Arabs in and driving Kurds out, starting in the late 1960s. The Kurdish leaders want to control Kirkuk and its oil wealth.

The Turkmen make a similar claim that Kirkuk is historically their city, and their backers in the Turkish government have warned the Kurds against any attempt to seize control. For the moment, Kurdish leaders are avoiding a showdown -- betting that a steady tide of Kurds returning to their old homes will gradually tip the demographic balance their way. One sign of the tension is that the sons of prominent Kurdish and Arab security officials were each kidnapped over the past week, according to the Iraqi press.

Into this ethnic cauldron jumped the U.S. military 16 months ago. American officials now play the Iraqi version of affirmative action -- keeping careful records of how many members of each ethnic group are on the local governing council, in the city administration and at each police station. They hope a new Property Claims Commission will pay Arab squatters enough compensation to go back home, but so far there are no judges here to arbitrate the claims.

The Kurdish police chief, Sherako Shaker Hakim, admits there are tensions on the force but tells me his rule is: "As long as someone is in Iraq, he's an Iraqi." The United States is encouraging this sort of unity by funding a "Joint Operations Center" for the Iraqi police and military. It makes for a nice photo opportunity -- with new computers and televisions and a diverse mix of police and military officers answering the phones. But a U.S. adviser here concedes that "it's a challenge every day" to maintain ethnic peace.

Kirkuk is an explosion waiting to happen. A violent move by any group -- Kurds, Turkmen or Sunni Arabs -- could detonate the mix. It's the political version of that car bomb roaming the streets. "I tell my soldiers one of their most important responsibilities is buying time," says Col. Milo Miles, who commands the Army brigade based here. And in that, he sums up a basic truth about Iraq.