U.S. and coalition airstrikes in December killed an estimated 2,500 Islamic State fighters in Iraq and Syria and another 60 in the first few days of January, a U.S. military spokesman in Baghdad said Wednesday.

"We're also hitting them in the pocketbook," said Army Col. Steve Warren, a spokesman for Combined Joint Task Force-Operation Inherent Resolve.

He said the coalition in the continuing Operation Tidal Wave II had carried out 65 airstrikes against oil facilities of the Islamic State of Iraq and Syria, or ISIS, in Syria from Jan. 2 to Jan. 6.

In addition, the airstrikes were increasingly focusing on northwestern Mosul, Iraq's second largest city which has been under ISIS since June 2014, Warren said in a video conference from Baghdad to the Pentagon.

In the last six months, the coalition has hit more than 700 targets in and around Mosul, including 191 in the last week, he said.

Warren and other U.S. officials have been increasingly outspoken in recent weeks about the progress of the campaign against ISIS since the White House last month revamped its messaging to offset the constant criticism of Congress.

"The Iraqi Security Forces are now on the offensive" since the retaking last week of the flashpoint city of Ramadi about 70 miles west of Baghdad along the Euphrates River, Warren said. ISIS has now lost about 40 percent of the territory it once controlled, he said. "The Iraqi Security Forces will pick when the next significant battle is," he said.

ISIS still controls the 60 miles of the river and the towns along it from Ramadi to the west to the town of Haditha, Warren said. The town itself and its nearby dam are held by Iraqi Security Forces, he said. Haditha and the dam had come under "harassing" attacks from ISIS in recent days, but Iraqi forces had repelled the fighters, he said.

Warren gave a brief explanation of how the U.S. estimates the number of enemy killed. He said U.S, advisors at Joint Operations Centers monitor video feeds from drones to gauge the number of enemy at a designated target before the airstrike.

For example, of six estimated enemy at a target, after the airstrike, if "nobody's moving -- scratch six," Warren said. Despite the losses, however, ISIS still has between 20,000 and 30,000 fighters in its ranks, he said.

--Richard Sisk can be reached at Richard.Sisk@military.com.