U.S.-led coalition forces in Iraq and Syria carried out a large-scale attack on Syria's Omar oil field as part of its mission to target the Islamic State group's ability to generate money, a coalition spokesman said Thursday.

Operations officer Maj. Michael Filanowski told journalists in Baghdad that airstrikes late Wednesday struck ISIS-controlled oil refineries, command and control centers and transportation nodes in the Omar oil field near the town of Deir el-Zour. Coalition spokesman Col. Steven Warren said the attack hit 26 targets, making it one of the largest set of strikes since launching the air campaign last year.

The refinery generates between $1.7 and $5.1 million per month for the Islamic State group.

"It was very specific targets that would result in long-term incapacitation of their ability to sell oil, to get it out of the ground and transport it," Filanowski said.

The Islamic State group seized a number of oil refineries and other infrastructure in Iraq and Syria as it sought to generate revenue to build a self-sufficient state. The group currently holds territory in just under a third of Iraq and Syria, and coalition forces continue to launch airstrikes in support of allied ground forces in both countries.

Coalition officials said that Iraqi security forces, backed by the paramilitary Popular Mobilization Forces and Iraqi federal police, continue to work to recapture and clear the western city of Ramadi and the city of Beiji, home to Iraq's largest oil refinery. They were being supported by airstrikes.

Iraqi forces said Tuesday that they had driven Islamic State militants out of Beiji and were in full control of the town. But coalition officials said Thursday that the mission has not been completed.

"Beiji city is contested," Filanowski said. "They control the road but then the next phase of the operation will be to clear the actual city."

Filanowski added that much of the area is littered with booby traps, and the militants have staged a number of counterattacks on advancing Iraqi forces as they look to control the town. He said that between 400 and 600 ISIS fighters are believed to have been fighting in Beiji — far less than the estimated 600 to 1,000 fighters in battling in Ramadi.

Meanwhile, a study released Thursday by IHS Jane's Terrorism and Insurgency Center said that from July through the end of September this year, the Islamic State group claimed a total of 1,086 attacks worldwide, with a 42 percent increase in the average daily number of attacks by the group.

The IHS report highlighted Islamic State activity across the area that it has claimed as its territory, including Egypt, Libya, Yemen, Afghanistan, Pakistan, Nigeria, Saudi Arabia, the North Caucasus and Algeria, in addition to the group's operational heartland in Iraq and Syria. The study considers an attack any act of aggression that causes casualties or disruption, including foiled attacks claimed by the militant group.

The report also noted that while most of the attacks happened in Iraq and Syria, Islamic State attacks in Nigeria during the third quarter yielded a higher death toll than those recorded in Syria, and had the highest average fatalities per attack of any of the group's self-declared states.