The two communities are increasingly frightened of each other because of bloody sectarian attacks over the past two years. So far the Shias have responded to repeated attacks on them by tit-for-tat killings but senior clerics have urged patience. That was tested by the death of the pilgrims in a fear-driven stampede on a bridge in Baghdad last week.

The powerful Shia radical cleric Muqtada al-Sadr suggested in a sermon on Friday that sectarian civil war had already started. He said: "We condemn the view that the [US] occupation's existence is beneficial for the Iraqi people because if it ended, there would be sectarian war - as if sectarian war had not already begun."

But Mr al-Sadr also praised the largely Sunni insurgents as "the noble resistance" and said Iraq was ruled by a colonial regime.

The rejection of the draft constitution by Sunni negotiators in the face of the determination of Shia and Kurdish leaders to push it through is also leading to greater hostility between Shias and Sunnis. The final draft of the constitution was accepted by the Iraqi parliament last Sunday amid allegations by Sunni leaders that it is a recipe for civil war. It will be voted on in a referendum on 15 October.

Also likely to increase political tensions is the announcement by the govern- ment that the trial of Saddam Hussein will start sometime between 15 and 20 October. One diplomat said the date was simply "posturing" by the government and the trial might take place at another date.

So far the Shia community appears numbed by the extent of the disaster on the bridge. One man was later killed and four others wounded in drive-by shootings aimed at two Sunni mosques in Zubair outside Basra, in the far south of the country. In Baghdad a march aimed at showing solidarity between Sunnis and Shias did not take place. At the big Um Al-Qura mosque, Sunnis far outnumbered Shias and a planned demonstration was cancelled.

Some Shias bitterly deny that Sunnis made any effort to help the dying and the injured during the disaster on the A'imma bridge last week during a Shia religious march to a shrine in the Khadamiyah district. A panic, sparked off by shouts that a suicide bomber was in the million-strong crowd, led to tens of thousands of people trying to run to safety.

As hundreds of people were buried at the end of last week tension was visibly rising between the communities. Ahmed Chasib, burying his wife Nadia Arif next to her sister, said that people in the strongly Sunni district of Adhamiyah had attacked the Shia marchers. "When we were near the bridge, the women ahead of us were hit by chemicals ... from Adhamiyah," he said. "Whoever told you that they helped us was a liar."

"We could have retaliated against the terrorists, but we do not want to be dragged into a sectarian war," said Sheikh Abdel-Mehdi al-Karabalai, who represents the top Shia cleric Grand Ayatollah Ali al-Sistani. Privately, however, some Shia leaders admit that retaliatory killings of Sunnis is going on. The killers are often police or police commandos and Sunnis are suspicious of the Interior Ministry, which is increasingly under Shia control. Some 36 Sunnis were found murdered in Sadr city after they had been arrested. The Interior and Defence ministers have blamed each other for the deadly stampede that killed 1,000 people.

The exact circumstances in which so many pilgrims were killed on Wednesday are still obscure. Even the numbers of dead are not known for certain.

The pilgrims were attending the mourning ceremony for a revered Shia saint who is buried beneath the golden comes of a shrine in the heart of Khadamiyah. The narrow streets of the district were not built to accommodate vast throngs of people. Under Saddam gatherings of this size were banned because he feared they would turn into political demonstrations. But since his overthrow these religious parades have become very popular.

Given the atmosphere of fear in Iraq it was always possible that there would be a mass panic during these religious ceremonies. Crowds of worshippers have been attacked by suicide bombers in Baghdad, Kerbala and Najaf over the past two years and hundreds killed. Six mortar bombs killed several worshippers close to the shrine on Wednesday.

Iraq is not yet split wholly along sectarian lines. Sunnis and Shias are often intermarried. The Kurds have been openly separatist for a century but the Shias in southern Iraq have never expressed general enthusiasm for autonomy from Baghdad, where they are in the majority. Outside Kurdistan, Iraqi nationalism is still a powerful force.

It is a mystery why the US should have put such faith in a new draft constitution as a major step forward in ending the fighting. It was rejected from the beginning by the Sunni community, which is the backbone of the insurgency. It is not only strongly federalist but envisages a weak Iraqi state in future. This will make it easier for the insurgents to present themselves as defenders of Iraqi nationalism.