Nadim Shehadi, an associate fellow at the Chatham House think tank, said countries such as Pakistan, Afghanistan and Saudi Arabia would risk being sucked into such a war.

But the academic said such a full-scale conflict between the two branches of Islam had not occurred before and religious leaders had a good track record of "calming" tensions in Iraq.

His comments came as new attacks were reported in the wave of violence that has killed over 140 people since the bombing of a revered Shia shrine in Samarra on Wednesday.

In one incident, gunmen broke into the house of a Shia family around 35 miles north of Baghdad and killed 13 people from three generations of one family. In Baghdad, bombers and gunmen targeted the funeral procession of Atwar Bahjat, the Sunni reporter gunned down on Wednesday while covering the shrine attack. At least three people were killed and six injured in the mayhem. Also, in the Shia holy city of Karbala, a car bomb killed at least four people and injured many others.

Shehadi said the current wave of violence was worrying but still fell short of a factional war sanctioned by religious leaders.

"A Sunni-Shia confrontation, if it becomes a declared conflict, will not confine itself to Iraq, it will spread as far as Afghanistan and Pakistan. So far it is being kept very much under control by religious leaders, but if it becomes a declared confrontation then everything that we have seen so far will look like a picnic."

However, Shehadi said other major sectarian incidents in recent years had not resulted in a full-scale war due to the influence of religious and tribal leaders.

He pointed in particular to the 2003 bombing by suspected Sunni insurgents outside a mosque in Najaf which killed more than 85 people, including a Shia leader.

"That could easily have resulted in a blood bath but it didn't because of the leaders calming it down, " he said.