An aid group has warned of an unfolding "human catastrophe" in Fallujah as Iraqi forces press to dislodge Islamic State militants from the city, where conditions are worsening for tens of thousands of trapped civilians.
IS fighters launched a fierce counterattack on the southern edge of the city, slowing the progress of the elite Iraqi counter-terrorism troops, and the militants reportedly corralled civilians into a single neighbourhood for use as human shields.
Iraqi forces repelled the four-hour counterattack a day after entering the southern part of Fallujah with the help of US-led air strikes. The dawn attack unfolded in the Nuaimiya area, most of which was captured by Iraqi troops on Monday, two special forces officers said.
IS militants used tunnels and snipers and targeted Iraqi forces with six explosives-laden cars that were destroyed before they reached their targets, the officers said. Iraqi forces suffered casualties but no details were given.
The clashes subsided by Tuesday afternoon but the officers said progress was slowed by roadside bombs the militants left behind. The troops also paused to destroy tunnels in the area. Since Monday 106 militants have been killed, the officers said.
The push into Nuaimiya, a sprawling agricultural area, was the first attempt by Iraqi forces to enter Fallujah, which fell to IS in 2014. In recent days, Iraqi forces had focused on expelling the militants from outlying areas to tighten a siege on the city, which is 40 miles (65 kilometres) west of Baghdad.
The Sunni majority city was the first in Iraq to fall to IS and is the last major urban area controlled by the extremist group in western Iraq. The Sunni-led militants still control the country's second largest city, Mosul, in the north.
The US-led coalition and Iranian-backed Shiite militia forces are helping the Iraqi army in the offensive.
But the fight is expected to be protracted because the militants have had more than two years to dig in, hidden bombs are believed to be strewn throughout the city, and the presence of trapped civilians will limit the use of supporting air strikes.
With an estimated 50,000 civilians still inside Fallujah, humanitarian groups renewed calls on both sides to open up safe corridors for non-combatants to flee - an action that seems unlikely because it would require negotiations between IS and the Iraqi forces to agree on a cessation of hostilities. In addition, Iraqi authorities want to prevent IS fighters from escaping the city by melting into the fleeing civilian population.
"A human catastrophe is unfolding in Fallujah," said Jan Egeland, head of the Norwegian Refugee Council.
Only one family managed to escape the town on Monday, he said. Since the offensive began more than a week ago, 554 other families have fled areas surrounding Fallujah.
"Warring parties must guarantee civilians safe exit now, before it's too late and more lives are lost," Mr Egeland added. A lack of food, medicine, safe drinking water and electricity is "pushing families to the brink of desperation", the NRC said.
At a briefing in Geneva, the spokesman for the UN refugee agency, William Spindler, cited figures by Iraqi authorities that said 624 families - about 3,700 individuals - have fled in the past week.
The UNHCR "understands another 500 men and boys over 12 years old are held for security screening", which can take five to seven days, Mr Spindler said.
"We understand some 27 men were released" on Monday, he said.
The 56-nation Organisation of Islamic Co-operation - the world's largest body of Muslim-majority countries - expressed "deep concern" about the safety of civilians in Fallujah. In a statement, the group's head also reiterated its opposition to sectarian violence and expressed support for the Iraqi government.
Some Sunni politicians in Iraq have accused the security forces of using indiscriminate force and say the Shiite militias have committed abuses against civilians in mainly Sunni towns and cities. The security forces and the militias deny the accusations.
The government-sanctioned umbrella group of mostly Shiite militias is not taking part in the current push into Fallujah, officials said, adding that the task is being handled solely by the elite counter-terrorism force.