Jim Michaels

USA TODAY

U.S.-backed Iraqi forces battling the Islamic State to liberate Mosul are suffering heavy casualties in the deadliest urban combat since World War II, according to top U.S. commanders for the Middle East.

Gen. Joseph Votel, the head of U.S. Central Command, said Wednesday that 774 Iraqi troops were killed and 4,600 wounded since the Mosul offensive began in October.

Tens of thousands of Iraqi troops are battling several thousand Islamic State militants holed up in their last major stronghold in Iraq.

The casualty statistics, released for the first time, highlight the difficulty of fighting in a densely populated city where the militants have had several years to build up complex defenses.

“This is the most significant urban combat to take place since World War II,” Lt. Gen. Stephen Townsend, the top coalition commander, said this week. “It is tough and brutal.”

The willingness of Iraqi armed forces to press ahead with the offensive despite heavy casualties is a remarkable turnaround after they collapsed nearly three years ago when the Islamic State invaded the country.

The militants swept into Iraq from Syria, capturing Mosul, in June 2014 in a devastating defeat for the armed forces. The militants seized U.S.-supplied arms and ammunition, while Iraq’s military melted away almost without firing a shot.

“Now they are a professional force,” said Army Col. Joseph Scrocca, a U.S. military spokesman. “The Iraqi security forces are putting themselves in the line of fire in order to protect civilians.”

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The dangerous urban combat has also exposed civilians to greater risk. The U.S. military said it is investigating a March 17 incident that killed dozens of civilians in western Mosul.

The U.S. military acknowledged an airstrike targeted the area, but officials also suspect that militants may have herded “human shields” into the building and might have stored munitions in the structure or rigged it to blow up. Townsend said the coalition used a small munition that was not designed to collapse an entire building.

The Pentagon said the air campaign exercises unprecedented caution to avoid civilian casualties, though avoiding deaths of citizens in a crowded city is difficult.

"As we move into the urban environment, it is going to become more and more difficult to apply extraordinarily high standards for things we are doing, although we will try," Votel told the House Armed Services Committee.

City fighting also places enormous challenges on ground forces. Fighting in urban terrain generally favors the defenders, who can place snipers in windows and hide down narrow alleys.

U.S. investigates Mosul strike after more than 100 civilians reported dead

Even with precision munitions, it is difficult to use air and artillery power in a dense urban battle. Much of the fighting falls on the shoulders of the individual soldiers, who have to clear the city block by block. Iraq's elite counterterrorism troops have been engaged heavily in the battle for Mosul.

Votel said 490 Iraqi forces were killed and 3,000 were wounded in the first phase of the offensive to liberate the eastern half of the city, which lasted about 100 days. Another 284 were killed and 1,600 wounded since the Iraqis began five weeks ago to retake the western half of Mosul.

The fight to drive the Islamic State, also known as ISIS, from the rest of the city will be difficult.

The U.S. military has deployed about 450 advisers to work with Iraqi forces in Mosul and are sending another 200 troops to the area, as Iraq increases its forces in the offensive. The Pentagon said the advisers are generally not near the front lines but work in headquarters behind the front lines.