The Iraqi army on Saturday began closing in on the last pocket of territory in Mosul held by the so-called "Islamic State" (IS) jihadist group, according to a military statement.

A day earlier, the Iraqi air force had dropped leaflets over the enclave warning residents to flee "immediately" to "safe passages." The United Nations said that as many as 200,000 civilians are believed to still be trapped and could try to leave, while aid groups fear that tens of thousands could be caught in the crossfire.

Read more: Mosul: The battle for Iraq's future

The move signals the potentially brutal operation that lies ahead. Since the Iraqi army launched its maneuver to recapture the country's second-largest city after Baghdad, officials warned civilians to remain indoors over fears of mass displacement. IS has also reportedly shot and killed fleeing civilians.

Since the Islamist group overran the city in 2014, as many as 100,000 civilians are estimated to have been killed.

The last IS pocket covers the Old City center and three adjacent districts by the western bank of the Tigris river. US-led coalition officials have said it was "only a matter of time" before the territory was recaptured.

Read more: Mosul's displaced: 'I'm at the end of my years and have to live in a tent'

Iraqi forces began their operation to reclaim Mosul back in October and declared the eastern side of the city "fully liberated" 100 days later.

Since the push into the western half of the city began in February, some 580,000 people have been forced to flee.

Stiff resistance

Resistance has remained fierce in the final 5 percent of the city held by IS - a territory of around 8 square kilometers (3 square miles).

During the liberation of Fallujah and Ramadi, cities that were also besieged by IS in 2014, Islamist fighters were forced to flee as their control over key territories dissolved. That allowed the Iraqi army to rack up many quick gains in the fight against the militant group.

In Mosul, however, IS fighters have found themselves encircled, preventing them from fleeing but allowing them to pin down Iraqi ground forces.

"Because the enemy cannot flee, the area is completely sealed off," Brig. Gen. Haider Fadhil of the Iraqi special forces said. "We are noticing that the closer we get to the Old City, the greater the resistance."

Read more: US plan to 'annihilate IS' raises questions over civilian toll, larger strategy

The decision to surround IS strongholds is part of a new military strategy introduced by US President Donald Trump's administration aimed at preventing militants from regrouping after suffering territorial losses and foreign fighters from fleeing and returning home.

"The foreign fighters are the strategic threat, should they return home," US Defense Secretary Jim Mattis said during a Pentagon briefing earlier this month. "So by taking the time to de-conflict, to surround and then attack, we carry out the annihilation campaign so we don't simply transplant this problem from one location to another."

dm/jlw (Reuters, AP)