Parts of Sydney's Muslim community have been accused of carrying out terrifying campaigns of boycotts, violence and unofficial policing of members of rival religious sects.

Community leaders fear an all-out sectarian war reminiscent of the Balkan proxy war of the 1990s as the conflict in Syria ignites tensions between Sunni and Shiite Muslims in Sydney. Some suburbs have ''no-go zones'' as death threats and verbal stoushes escalate into firebombings, bashings and shootings.

Assistant Commissioner Frank Mennilli said police were aware of several cases of extortion and violent threats made against Shiite- and Alawite-owned businesses after 23 were placed on an online boycott list. ''In terms of reported cases it is a very small number; however, police are aware of several unreported incidents and have responded accordingly,'' he said.

The civil war in Syria pits the Sunni-dominated rebellion against the regime of President Bashar al-Assad, whose government has supported Syria's Alawite Muslims and Shiite Muslims in Lebanon and Iran.