Iraqi counterterrorism forces face off with Islamic State militants on the southern edge of Fallujah, Iraq, on Tuesday, a day after launching an operation to retake the militant-held city with the help of U.S.-led coalition airstrikes.

An Iraqi counterterrorism soldier stands next to his Humvee during clashes with Islamic State militants on the southern edge of Fallujah, Iraq, on Tuesday.

Iraqi counterterrorism forces face off with Islamic State militants on the southern edge of Fallujah, Iraq, on Tuesday, a day after launching an operation aimed at the militant-held city with the help of U.S.-led coalition airstrikes.

Iraqis gather water in a neighborhood in central Fallujah, Iraq, as Iraqi counterterrorism forces battle Islamic State militants on the southern edge of the city on Tuesday. A leading aid group raised alarm over the unfolding "human catastrophe" here, where an estimated 50,000 people remain trapped as the fight intensifies.

An Iraqi counterterrorism soldier perches on a berm during clashes with Islamic State militants on the southern edge of Fallujah, Iraq, on Tuesday.

Iraqi counterterrorism forces fix their armored vehicles and load their weapons during a break in fighting outside Fallujah, Iraq, on Tuesday. The elite troops repelled a four-hour attack by the Islamic State group in the city's south a day after first moving into the southern edges of the militant-held city with the help of U.S.-led coalition airstrikes.

In this image posted on a photo sharing website by an Islamic State militant media arm on Monday, a military vehicle burns as Islamic State fighters battle Iraqi forces and their allies west of Fallujah, Iraq.

In this image posted on a photo sharing website by an Islamic State militant media arm on Monday, an Islamic State fighter takes aim during battle with Iraqi forces and their allies west of Fallujah, Iraq.