(CNN)Nearly 19,000 civilians were killed in Iraq between January 2014 and October 2015 -- a toll the United Nations calls "staggering" in a new report.

The report, released Tuesday, outlined the horrific impact that Iraq's ongoing conflict is having on its civilian population.

Much of the suffering was attributed to ISIS, the brutal Islamist terror group which has declared an Islamic caliphate across the vast stretches of territory it holds in Iraq and neighboring Syria.

The report also estimated that ISIS holds about 3,500 slaves and said it continues to subject women and children to sexual violence, particularly in the form of sexual slavery.

Other numbers in the report are mind-boggling. In the 21-month period in Iraq:

• At least 18,802 civilians were killed, about half of them in Baghdad.

• Another 36,245 were injured.

• About 3.2 million people were internally displaced, including a million school-aged children.

The actual figures could be much higher, the report said.

The report also documented alleged abuses by Iraqi security forces and allied groups fighting ISIS, but attributed much of the suffering to ISIS itself.

"The so-called 'Islamic State of Iraq and the Levant' (ISIL) continues to commit systematic and widespread violence and abuses of international human rights law and humanitarian law. These acts may, in some instances, amount to war crimes, crimes against humanity, and possibly genocide," the report said, using another name for ISIS.

The group, which has controlled Iraq's second largest city, Mosul, since June 2014, employed horrifying methods of killing, the report said, including beheading, bulldozing, burning alive and throwing people off the tops of buildings.

However, improvised explosive devices -- including explosives worn by suicide bombers and those carried in vehicles -- were the deadliest tactic used against civilians, it said.

The report was prepared by the U.N. Assistance Mission for Iraq and the Office of the High Commissioner for Human Rights and was based largely on testimony obtained directly from survivors or witnesses of rights violations, including interviews with internally displaced people.

The U.S. will deploy "certainly hundreds" more troops to Iraq to train additional brigades which Prime Minister Haider al-Abadi has pledged will take back Mosul from ISIS this year, a U.S. military spokesman in Baghdad said Wednesday.

The extra troops will inevitably boost the costs of the campaign against ISIS in Iraq and Syria that totaled $5.53 billion through Dec. 15, or about $11 million daily, said Army Col. Steve Warren, a spokesman for Combined Joint Task Force-Operation Inherent Resolve.

Most of the costs have been incurred by the air campaign. Warren said the U.S. and coalition allies have flown 65,492 sorties to date, including 9,782 airstrikes. A total of 6,516 of those airstrikes were in Iraq, and 3,266 in Syria, he said.

Warren said that the new deployment of trainers will be in addition to the 1,300 soldiers from the 2nd Brigade Combat Team of the 101st Airborne Division out of Fort Campbell, Kentucky, who will deploy to Iraq this spring to replace the 1st Brigade Combat Team of the 10th Mountain Division in the training role. The 1st BCT will return to Fort Drum, N.Y.

"The reason we need new trainers, or additional trainers, is that's really the next step to generate the combat power needed to liberate Mosul," Warren said. "We need more (Iraqi) troops trained in more specialties," such as commandos and snipers, to mount a push north from Baiji up the Tigris River valley to attack Mosul, he said.

"We don't have a solid number yet" on how many new trainers will deploy to join the estimated 3,550 U.S. troops now on the ground in the campaign against ISIS, Warren said. "It's certainly hundreds -- that will probably be at the top end – not thousands, hundreds," but additional U.S. support troops may also be needed for the Mosul buildup, he said.

The number of new U.S. troops will also be conditional on the number of additional trainers U.S. allies were willing to provide, Warren said.

WASHINGTON -- The Pentagon said Wednesday that it spent less than $10 million -- far less than previously reported -- on a controversial natural gas filling station in Afghanistan.

The station was the focus of an inspector general investigation into wasteful spending but a consultant that put forth the original $43-million estimate has now dramatically revised that figure, said Brian McKeon, principal deputy under secretary of defense for policy.

John Sopko, the special inspector general for Afghan reconstruction, accepted the new estimate but said even $10 million was unjustified for the project.

Both men were called to testify before the Senate Armed Services Committee on Wednesday after SIGAR published reports in recent months finding a $800-million Defense Department task force charged with rebuilding the Afghan economy had wasted money on an ill-conceived natural gas facility in the northern town of Sheberghan and also spent $150 million on lavish accommodations such as flat-screen TVs and personal refrigerators.

“We believe the methodology used by [the consultant] is flawed and that the station costs are far lower,” McKeon testified.

After a review, the firm told the DoD that the total costs are likely “well under $10 million.” The cost estimates changed because SIGAR calculated $30 million in overhead costs that actually were tied to a variety of other energy extraction projects in Afghanistan, he told lawmakers.

Sopko, who has rooted out numerous examples of potentially embarrassing waste, defended his release of the much higher estimate in a report published in October.

“It was the best evidence that we had at the time … it would have been irresponsible for SIGAR not to report it,” Sopko said.