The seizure of five Britons in Iraq is ominous. The Government is right to take this latest outrage extremely seriously and convene a meeting of Cobra, the Whitehall emergency response committee. Early reports suggest that the kidnapping of security guards was carefully planned and carried out by armed Iraqis wearing police uniforms. Given the continuing high level of violence, the daily demands for ransom and vicious treatment of Iraqis seized by sectarian militias and the disappearance and murder of other Westerners who have been kidnapped, the Government must brace itself for difficult decisions and perilous consequences.

It is unclear at this stage whether the seizure is politically or criminally motivated. The Iraqi police are notoriously corrupt, and many in Baghdad fear that they have been infiltrated by extremists and rival militia factions. The police have failed to halt the many sectarian kidnappings that have struck terror into middle-class families and paralysed normal life in the city, with children kept from school and some people afraid to go out into the streets. Indeed, it is sometimes the police themselves who are behind the seizures. Those Western security guards who are employed to protect Western advisers and businessmen in Baghdad are an obvious target: not only do criminals calculate that they can extort large ransoms for their return, but extremists linked to al-Qaeda know that if security firms are vulnerable, the pressure on all outsiders to leave will grow and so will the despair of ordinary Iraqis.

Most security guards, recruited from former special forces personnel, are well versed in the risks. The job is obviously lucrative but patently dangerous. Until yesterday only three Britons have been snatched since 2003. But negotiating their release becomes ever more difficult, especially as previous rescue attempts, including the daring military mission that rescued Norman Kember last year, make the kidnappers warier and more intransigent. British troops have contingency plans for new seizures, but would be foolish to provide details of how or whether a rescue will be attempted.

The kidnapping, which took place inside the Finance Ministry when police vehicles blocked the street and armed men stormed a seminar, comes as violence continued throughout Iraq. Car bombings left more than 38 people dead in Baghdad, the bodies of 21 people were found shot in two districts 40 miles north of the capital, the editor of a newspaper in Kirkuk was killed and at least eight US service personnel died when an American helicopter crashed in Diyala province and a rescue team was ambushed by roadside bombs. The Americans have lost 112 troops this May, the highest monthly toll since December.

The seizure also came a day after the talks in Baghdad between US and Iranian officials on ways to stabilise Iraq. American negotiators warned Iran that it should stop supplying weapons to Shia militias and to other terrorist groups for use against coalition forces. Iran now has a good opportunity to demonstrate good faith by using its considerable influence among Shia militias to track down the kidnappers and persuade them to release their victims. Tehran must be as concerned as Western governments that these attempts to make Iraq ungovernable will only hasten an all-out civil war. That would be disastrous for Iraq, for Tehran and for the entire region.