Leading article: The civil war that looks ever harder to avert

Baghdad's relative security has always been deceptive. While the heavily guarded Green Zone has shielded the government and foreign representations from attack, what is euphemistically termed low-level violence has simmered elsewhere. The north-eastern suburbs, once known as Saddam City, now renamed Sadr City, have often borne the brunt of the killing.

Yesterday was different. Gunmen identified as being from a Shia militia went on the rampage through the Jihad district of western Baghdad, picking out Sunnis and killing them at close range. More than 40 Sunnis died in this shooting spree. Later, another 17 people were killed in two car bomb attacks near a Shia mosque.

Bands of gunmen moving systematically through a district, killing civilians from another religious group, is something that has been seen in mixed neighbourhoods outside Baghdad, but never before on this scale in the capital. The immediate explanation given by officials was that the shootings were in retaliation for a car bomb outside a nearby Shia mosque the day before. Others saw the rampage as the latest flare-up in a Shia-Sunni struggle that has targeted mosques elsewhere in Iraq since February, when a Shia shrine at Samarra was bombed.

Murderous and destructive though both are, however, there is a qualitative difference between bombings that target mosques and the systematic shooting of civilians in their cars or in their homes. The faithful can assess the risk of visiting the mosque and stay away. No one can guard against his car being stopped or his home invaded, just because he happens to be from a particular ethnic or religious group. This is sectarian, inter-communal violence at its most primitive and threatening. No one can feel safe in such a climate.

The latest developments can only be a source of huge discouragement to the Prime Minister, Nuri al-Malaki, not to speak of the US and British governments and the many Iraqis hoping for peace. The appointment of a Prime Minister and the long-drawn-out negotiations to form a government constituted the final stage of Iraq's post-Saddam transition. The hope, widely shared, was that with all the components of a representative government in place, Iraq would finally settle down and bury its differences in the name of national reconstruction.

Hopes were reinforced last month, when a US missile attack killed the presumed head of al-Qa'ida in Iraq, Abu Musab al-Zarqawi, who was held responsible for some of the most heinous killings of foreigners. The thinking, especially among US officials, was that without Zarqawi, whatever part of the violence was inspired by al-Qa'ida would quickly subside. The US had long preferred to view the violence in Iraq as terrorism, rather than anti-occupation or sectarian in origin.

The new solutions, such as there are any, look even less promising. An elaborate security plan for Baghdad, devised by the Iraqi government, with the backing of the US, has yielded disappointing results; it looks set to be revised or abandoned. And yesterday representatives of nine states in the region, meeting in Tehran, gave their backing to an Iraq "reconciliation plan", while declining to offer direct mediation. Iraq finds itself in a Catch-22, where violence renders the authorities powerless and the impotence of the authorities fosters violence.

Yesterday's shootings mark another stage in the descent of Iraq into civil war. If Shia elements have taken up arms against Sunnis, simply because they are Sunnis, and if they have been able to take their fight to the capital, then it is hard to see how, or where, the tide of violence can be turned back.