Islamic State militants were producing about $2 million worth of crude oil per day in Iraq and Syria before recent U.S.-led air strikes, possibly more than double the amount heard in U.S. congressional testimony last month, the research firm IHS said on Monday.

The Sunni extremist group controlled as much as 350,000 barrels per day of crude oil, but was able to produce only about 50,000 bpd to 60,000 bpd, said IHS, a provider of global market and economic information.

"This fraction of pre-war capacity is the result of warfare, shut-ins and (Islamic State's) limited technical prowess operating the fields," IHS said in a paper called "Spoils of War: Who's in charge of what oil resources in the conflict zones of northern Iraq and Syria."

U.S.-led air strikes in September hit Islamic State oil refineries in eastern Syria as Washington and its partners aim to choke off an important source of revenues for the militant group.

Similar air strikes hit a modular oil refinery near Deir al-Zor, south of Kobani, Syria, the U.S. military said last week.

IHS said some of the militants' capacity to produce oil has almost certainly been disrupted by the air strikes, but it was too soon to tell how much.

Last month, Nicholas Rasmussen, deputy director of the National Counterterrorism Center, testified to Congress that the Islamic State's war chest probably included about $1 million a day in revenues from black-market oil sales as well as smuggling, robberies, and ransom payments for hostages.

Most of the oil sold by the militant group on the black market is moved via trucks along smuggling routes on the Turkish border, IHS said. It is sold at a steep discount, ranging between $25 to $60 a barrel, compared with about $85 per barrel for international benchmark Brent oil.

IHS said it is unclear after the U.S.-led air strikes whether Islamic State has enough refining capacity, which consists mostly of simple mobile plants that can be loaded and transported by truck, to meet its own needs.