CAMP TARIQ, Iraq — As Iraqi forces pressed an offensive Tuesday to dislodge Islamic State militants from Fallujah, conditions are worsening for tens of thousands of civilians trapped in the city, and a leading aid group raised alarm over an unfolding “human catastrophe.” Islamic State fighters launched a fierce counterattack on the southern edge of the city, slowing the progress of the elite Iraqi counterterrorism troops, and the militants reportedly corralled civilians into a single neighborhood for use as human shields. The troops also paused to destroy tunnels in the area. Since Monday, 106 militants have been killed, the officers said. With an estimated 50,000 civilians still inside Fallujah, humanitarian groups renewed calls on both sides to open up safe corridors for noncombatants to flee — an action that seems unlikely because it would require negotiations between Islamic State and the Iraqi forces.