When Iraq's top generals finalized the plan to retake Mosul from the Islamic State group, they gave themselves six months to finish the job.

"It was the maximum time cap," Prime Minister Haider al-Abadi said last week. "We had to plan for the worst, so we don't get surprised."

Six weeks into the battle, the force made up of 50,000 troops, Shiite and Sunni tribal militias and Kurdish fighters is a long way from winning back the country's second-largest city. The fight is showing the limitations of Iraq's military and security forces, suggesting it has still not fully recovered from the collapse it suffered two years ago in the face of the militants' blitz across much of northern and western Iraq.

As expected, IS militants are tenaciously defending their last major foothold in Iraq, and the 1 million civilians who remain inside prevent the use of overwhelming firepower.

But what is alarming, according to Iraqi field commanders, is that the progress so far has been lopsided. The battle-seasoned special forces are doing most of the fighting and slowly advancing inside the city. Other military outfits are halted outside the city limits, unable to move forward because of resistance, battle fatigue, inexperience or lack of weaponry suited for urban warfare.

Another major challenge for the Iraqis is the command of large and disparate forces maneuvering for a coordinated assault on a large city, according to retired U.S. Army Lt. Gen. Mark P. Hurtling, the top American soldier in northern Iraq during the troop surge of 2007-2008.

"This will continue to be a tough fight," he said. Many of the Iraqi army commanders are seasoned, but "most of the soldiers are young, new and have not experienced combat."

The special forces are under considerable pressure to push on, slugging it out through treacherously narrow streets and alleys while enduring a daily barrage of suicide bombings and mortar and rocket shells.

As policy, the Iraqi military does not release casualty figures, but special forces' officers speak privately of scores killed and wounded.

"We must continue to advance because we suffer fewer casualties than if we hold still and wait for other units to advance in their sectors," said Brig. Gen. Haider Fadhil of the special forces. "We are trying to advance cautiously to minimize casualties, and we are convinced that we will eventually be asked to liberate the western sector of the city when we are done here."

That may be a while yet.

The special forces have driven IS militants from about 15 of eastern Mosul's estimated 39 neighborhoods, some of which are no more than a handful of blocks. Their progress to date places them about 3 kilometers (nearly 2 miles) from the Tigris River, which divides the city in half.

It's a deceptively short distance: The area is densely built up and heavily populated, and the men are advancing on multiple fronts, constantly assigning valuable resources to securing their flanks and rear as they capture more territory. For example, in a week of fighting, they have only taken about 60 percent of the large and densely populated Zohour neighborhood, site of one of Mosul's busiest food markets.

Mosul's eastern half has a greater population than the western half. In a positive note, coalition airstrikes that cut off the city's four bridges across the river have helped reduce the number of car bombs, commanders say.

In contrast, the regular forces, which have been battling for weeks through towns and villages on the way to the city, are now stalled on the edges, facing the prospect of diving into an urban battle, according to commanders in the field.

The 16th Infantry Division met stiff resistance about 10 kilometers (6 miles) north of the city and has had to halt its advance and hold its positions. It did send a brigade to the eastern side of Mosul to help hold territory taken by the special forces there. South of Mosul, forces from the 9th Division along with some 10,000 members of the paramilitary federal police have stopped about 4 kilometers (2.5 miles) short of the city's boundaries to regroup and rest after the long fight to reach that point.

"We are not equipped or trained to fight inside cities. We are an armored outfit with tanks," acknowledged a senior officer from the 9th Division, who spoke on condition of anonymity in line with military regulations. "Even if we are able to advance toward the city, we need forces to hold the territory we liberate. Our men are exhausted after a six-week battle deployment."

The IS militants are putting up resistance not seen in past major battles with the Iraqi military north and west of Baghdad. They are drawing on a vast arsenal of weapons, while fighting in a city they called home for the past two years. The militants have also dug an elaborate network of tunnels, some as long as 3,000 meters (yards), that offer cover from drones.

The battle "will require a combination of siege warfare and continuous attacks against targets identified by sophisticated intelligence, all taking place while civilians are still in the city and its surroundings," Hertling wrote in response to questions emailed by The Associated Press. "It will take longer than any of them have predicted, and they will sustain significant casualties."

Commanders suspect the worst may yet to come.

Up to 6,000 IS fighters remain in Mosul, including as many as 1,500 foreign fighters, with French and Belgians believed to number several hundreds, according to a senior military intelligence officer in Mosul.

"Their escape from Mosul is now difficult, so they will either fight to the death or become suicide bombers," said the officer, who spoke on condition of anonymity in line with military regulations.

IS also appears to use its own drones to gather intelligence on troop movements. An example of their information-gathering abilities came Thursday, when fighters accurately hit with several mortar rounds a tent near the airstrip outside the town of Tal Afar, west of Mosul. The attack took place just minutes after the prime minister concluded a meeting there with leaders of the Shiite militias tasked with liberating Tal Afar.

Two senior militia leaders and four members of their security details were hurt, according to Jaafar al-Husseini, a militia spokesman.