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.
IS claims to be successfully repelling the Iraqi army offensive against it in Falluja, providing battle footage and images as evidence.
Last week, the group highlighted what it said was the impact of the offensive on civilians in the city, releasing footage of severely-injured children that was then widely shared on social media. The group typically shows images of civilian casualties to undermine anti-IS military campaigns.
Although IS has not commented on civilians trapped in the city in the latest offensive, it has in the past warned Sunni Muslims against leaving IS-held territory.
And last week its spokesman, Abu-Muhammad al-Adnani, said that IS militants would “fight to the death” in all its strongholds even if it meant the death of civilians – indicating that the group will pursue a scorched earth policy.
As such, it is unlikely that IS would allow civilians to leave Falluja. The UN refugee agency says families have reported being used as human shields by IS, forced to move with militants from one location to another.
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.