Red Cross workers were permitted to briefly enter Falluja for the first time since a joint US-Iraqi offensive began against insurgents there.
The International Committee of the Red Cross said seven local staff members entered the city last week.
They found severely disrupted water and sewage services, and were concerned at an apparent lack of medical services.
A spokesman said that although the team saw little sign of civilians, they believe people are still in the city.
"Seven of our local colleagues went into Falluja, into the center of the city, for a few hours," said Florian Westphal.
They met Iraqi water and sewage officials and visited a makeshift clinic inside a mosque, he added.
He said they saw sewage flowing through the streets, and were concerned at the lack of medical facilities available.
Although they saw few civilians, it is thought people are still in the city but staying indoors.
The Iraqi officials told the Red Cross group "the water treatment facilities and sewage systems had been damaged and are currently not functioning," Westphal told AFP news agency.
The ICRC says restoring water supplies will be its main priority in Falluja.
This trip is the first the International Committee of the Red Cross has made into the city, although the Iraqi Red Crescent has already been in on short visits.
Tens of thousands of civilians fled when US and Iraqi troops went in to the city to fight insurgents. They are reportedly now preparing to return.