The language was stark. Iraq’s slide towards chaos could spark “a broader regional war”, according to the blue-chip panel reporting to President Bush this week. There was a risk of “regional conflagration”, said Robert Gates, the incoming Pentagon chief, the day before.

Yet even as these dire warnings were being delivered in measured tones by Washington’s wise men, there are disturbing signs that Sunni-Shia violence is already bleeding across Iraq’s borders.

If the sectarian strife spreads, the Iraq Study Group cautioned on Wednesday, neighbouring countries face instability as the two Muslim sects vie to protect their spheres of influence. “Ambassadors from neighbouring countries told us that they fear the distinct possibility of Sunni-Shia clashes across the Islamic world,” the Iraq Study Group, led by James Baker and Lee Hamilton, wrote.

Their alert may have been buried deep within the 100-page report. But it is page one, paragraph one of neighbouring regimes’ concerns, as the report recognised: “Such a broader sectarian conflict could open a Pandora’s box of problems — including the radicalisation of problems, mass movements of populations and regime changes — that could take decades to play out.”

For Ahmad Mahmoud, it did not take decades. The 20-year-old ’s face now stares impassively from mourning posters plastered on his two-storey home in Beirut. A Shia, he lived in the mainly Sunni neighbourhood of Tarek Jdeide. Mahmoud was shot dead on Sunday during street clashes between Sunnis and Shias.

It was the first fatality of Lebanon’s worsening political crisis, which has soured the already tense relations between the two communities. His death sparked further clashes in Beirut, where for the first time the Sunni-Shia split was overshadowing the more traditional divide between Christians and Muslims.

The sectarian tensions behind Mahmoud’s death are being played out across the Middle East, where broader suspicions and hostilities have led to a riot in Damascus suburbs, a knife fight between pupils and cruel barbs in Amman schools. In Syria and Jordan — both Sunni countries — there is rising domestic anger at the daily slaughter of their brethren in Iraq, which is beamed into their homes by satellite television. Many blame Shia Iran for stoking the conflict there.

Many regimes also fear that al-Qaeda’s brand of Sunni militancy will spread after thousands of young Arabs who fought with the insurgents in Iraq return home, emboldened to take on their leaders. King Abdullah II of Jordan has talked, separately, of a “Shia Crescent” from Iran to Lebanon, and last week cautioned that the region could end up with three civil wars: in Lebanon, Iraq and the Palestinian territories.

“Iran is now connected to all the ongoing conflicts in the region in Iraq, in Lebanon, and in Palestine, and is now the main address you have to go through for solving all these conflicts,” a senior Jordanian official said.

“[Iran] is operating through its local allies, in Iraq, and through Hezbollah and Hamas. It is engaging with the marginalised Shia communities in Saudi Arabia, Bahrain and in Kuwait to turn them into arrows that can challenge their Government’s legitimacy and authority.”

The growing regional influence of Iran was certainly a dark undercurrent beneath last week’s Amman mini-summit between Mr Bush and Nouri al-Maliki, the Iraqi Prime Minister. There are fears among Sunni regimes that Tehran is distracting US efforts to curb its nuclear ambitions by using its oil wealth and regional proxies to foster trouble across the region.

Another potential cause for instability is the more than one million Iraqis who have fled into Syria and Jordan. In June, a fatal quarrel between a Syrian and an Iraqi refugee triggered rioting in Geramana, an Iraqi-majority suburb south of Damascus. Witnesses said that scores of Syrians took to the streets, burning Iraqi cars and shops and three Iraqi-run real estate offices before they were dispersed by police.

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