Militants from the so-called Islamic State have launched a dawn counter-attack as Iraqi government troops push into the city of Falluja.
A day after troops advanced through the southern suburb of Nuaimiya, scores of IS fighters attacked them, army officers told news agencies.
The army defeated the attackers but suffered casualties, the sources said.
Aid workers are increasingly concerned for the safety of 50,000 civilians said to be trapped in Falluja.
Reports speak of people starving to death and of being killed for refusing to fight for IS.
The Iraqi military has urged those remaining to either leave the city or stay indoors but IS is preventing civilians from fleeing.
Falluja fell to IS in January 2014, a key moment in the Sunni Muslim jihadist group's rise that saw it declare a caliphate across swathes of Iraq and Syria.
It is one of two major cities held by IS in Iraq – the other being Mosul.
'They came at us'
Lt Gen Abdelwahab al-Saadi, the overall commander of the Falluja operation, told AFP news agency around 100 IS fighters had taken part in the attack and 75 of them had been killed.
"They came at us heavily armed but did not use car bombs or suicide bombers," he said.
But two officers with special forces in the area told the Associated Press news agency that the jihadists had sent out six explosives-laden cars, none of which reached their targets.
IS also used snipers and tunnels in the attack, they added, speaking on condition of anonymity.
Jan Egeland, secretary general of the Norwegian Refugee Council, which helps families displaced from the city, warned on Tuesday that a "human catastrophe" was unfolding in Falluja.
"Families are caught in the crossfire with no safe way out," he said.
"Warring parties must guarantee civilians safe exit now, before it's too late and more lives are lost."
The situation of civilians in the city is dire, the BBC's Jim Muir reports from Iraq.
They have lived through a nine-month siege with little food or medicine and now they face mortal danger if there is an all-out assault on the city centre, our correspondent says.
The IS fighters are being widely accused of using them as human shields, he adds.
Falluja – key facts:
- had population of more than 300,000 before it was seized by first al-Qaeda and then by IS in 2014
- has always been a hotbed of Sunni defiance, becoming a symbol of resistance to US forces in violent battles in 2004
- controls main highway from Baghdad to both Jordan and Syria
- is known as the "city of mosques" – more than 200 mosques in city and surrounding area
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