The militias are fighting alongside Syrian government forces to keep Assad in power. But officials think Iran's long-term goal is to have reliable operatives in Syria in case the country fractures into ethnic and sectarian enclaves, the Washington Post reported.
A senior Obama administration official cited Iranian claims that Tehran was backing as many as 50,000 militiamen in Syria.
"It's a big operation," the official said. "The immediate intention seems to be to support the Syrian regime. But it's important for Iran to have a force in Syria
that is reliable and can be counted on," the official added.
Iran's strategy, a senior Arab official agreed, has two tracks. "One is to support Assad to the hilt, the other is to set the stage for major mischief if he collapses."
The fragmentation of Syria along religious and tribal lines is a growing concern for neighbouring governments and the administration, as the civil war approaches its third year with little sign of a political solution or military victory for either Assad's forces or the rebels, the report said.
Rebel forces, drawn largely from Syria's Sunni majority, are far from united, with schisms along religious, geographic, political and economic lines. Militant Islamists, including many from other countries and with ties to al-Qaeda, are growing in power, it said.
Kurdish nationalists have their own militias, with control over major swaths of the northeastern part of the country and in parts of Aleppo. They are far more interested in autonomy than in an alliance with either side in the conflict, it added.
Minority Christians have largely sided with Assad, fearing the outcome of an Islamist victory. Syria's 700,000 Druze, followers of an offshoot of Shiite Islam, are increasingly leaning toward the rebels, the paper said.
Despite US efforts to convince members of Assad's Alawite sect, itself a minority within Islam's Shiite branch, that their interests lie in abandoning him, Alawite support remains fairly solid, it said.