Islamic State militants now world's richest: experts

US officials say the Islamic State has become the world's wealthiest terror group.

The group generating tens of millions of dollars a month from black market oil sales, ransoms and extortion and $1 million a day alone by selling crude oil from fields it captured across Iraq and Syria earlier this year, according to David Cohen, Treasury undersecretary for terrorism and financial intelligence.

Earlier this month France's foreign minister said the Islamic State was selling oil to the government of Bashar al-Assad in Syria.

A Wall Street Journal article describes how IS smuggles the oil out:

The route begins with oil fields run just a few years ago by Western energy giants and now controlled, along with fuel-smuggling operations in Syria, by the Islamic State.

The militants truck oil drawn from those fields or stolen from pipelines to rudimentary refineries... The refined products are sent to the Turkish frontier, where they're hauled over the border by trucks, horses or mules, according to these accounts. Fuel has also been floated across rivers on rafts or pumped through underground pipelines before finding its way to markets across southern Turkey.

Initially, Turkey largely turned a blind eye to the illicit fuel trade, which ramped up at the start of the Syrian uprising in 2011 along smuggling routes that have existed for decades.

Crude from these fields, as well as oil stolen from tapped pipelines and from other fields across the country, is processed into low-quality fuels, including diesel, in a number of makeshift refineries in Islamic State-controlled Raqqa province