(CNN) It was a very bad day for Iraqi forces and Peshmerga fighters leading the offensive to liberate Mosul from ISIS.

When the long-awaited offensive began October 17, Iraqi Prime Minister Haider al-Abadi vowed victory but warned the effort could take time.

If a pitched battle that CNN's senior international correspondent Arwa Damon and photojournalist Brice Laine were caught in Friday and Saturday is a sign of fights to come, the offensive to take back Iraq's second city will only get more difficult.

Defeating ISIS in Mosul would represent a major victory for the Abadi government as it struggles to boost its credibility, prove its military prowess and end the terrorist group's territorial dominance in Iraq's oil-rich north.

Altogether, the Iraqi force is about 30,000-strong, and includes troops from elite units such as the Counter Terrorism Force and the Golden Division. These were among the same units involved in heavy fighting in the liberation of Falluja and Ramadi.

But they're not as strong as they once were. They come to battle with a mix of qualities and capabilities among regiments.

And they have poor communications. A spokesman for one division said the headquarters can't talk directly to most units on the battlefield. The inability to maintain constant communication presents a huge problem in an urban environment.

JUST WATCHED Inside shocking gun battle in Mosul Replay More Videos ... MUST WATCH Inside shocking gun battle in Mosul 03:17

An agile, fearless enemy

ISIS, an agile enemy, has been preparing its defenses for two years. It takes advantage of the terrain, a network of tunnels and booby-trapped buildings, to great effect.

US military officials estimate there are 3,000 to 5,000 ISIS fighters in Mosul. An additional 1,500 to 2,000 ISIS fighters may be waiting outside the city limits.

The streets of Kirkukli, a neighborhood on the eastern outskirts of Mosul, are tight and narrow. ISIS' style is not to challenge Iraqi forces on open ground, which would make them inviting targets for air power. So they sit and wait for Iraqi forces to move into densely populated areas. Then they pounce.

The use of civilians as shields makes it difficult for Iraqi forces to fire at snipers. ISIS has mastered this classic tactic of guerrilla warfare.

And many ISIS fighters, particularly those left behind in areas east of Mosul, know they're going to die. Fighters are known to pop out from behind buildings, just yards from approaching Humvees, to unleash rocket-propelled grenades. In addition, the city's eastern outskirts are dotted with cars rigged to be ISIS car bombs.

The army faces hurdles

That the Iraqi forces don't know Mosul does not help. Most of them are Shia. They're fighting in unknown territory, a largely Sunni-Arab city that most have never visited.

Friday, they moved straight into an ambush, with firepower descending on them from three sides.

When a lumbering Humvee at the front of the pack was hit, the ones behind it became sitting targets. Of the 15 Humvees that went out with the unit in Kirkukli, only three drove out. Some had to be towed. Others were abandoned.

Reinforcement is difficult to arrange. Not only are the communications poor, but the troops are depleted.

A relief convoy took eight hours to travel about two kilometers (1.25 miles). Then it, too, became trapped as it came under fire.

Saturday morning, another convoy of about 30 Humvees arrived, but the hulking vehicles are vulnerable in open combat to rocket-propelled grenades and mortars.

The units survived the skirmish with one fatality and 20 wounded. Saturday, they made progress in other areas near Mosul.

But if this is the resistance ISIS put up on the edge of Mosul, expect the fight to intensify as Iraqi forces push into areas where the terrorist group moves through tunnels and has rigged buildings with bombs.

Thousands displaced by fighting

Mosul is Iraq's second most-populous city, with 1.2 million residents. Aid agencies talk about the possibility of nearly 1 million refugees.

There are 28,890 people currently displaced by the offensive to retake the ISIS stronghold of Mosul that began October 17, the International Organization for Migration said.

Although the mass exodus feared by the United Nations has yet to materialize, the number of displaced Mosul residents has grown sharply in recent days. From Friday to Saturday, the IOM registered more than 6,500 new refugees in what the UN Refugee Agency is calling "the largest displacement of people from Mosul since the start of the offensive."

The Khazer camp was built with 1,000 tents meant for 8,000 people. It has been the site of emotional reunions between residents of Mosul and family members who haven't seen them during two years of ISIS rule.

The United Nations said that camp and another site in Irbil saw about 8,000 people in the past three days.

"People arrived with nothing or next to nothing," the UN's Frederic Cussigh said.

The United Nations has five refugee camps and plans to build six more.