Civilians caught up in fighting between security forces and Shi'ite militiamen in a Baghdad slum are running out of food, water and medicines and relief agencies are unable to bring in supplies, officials said on Thursday.

Aid officials and an Iraqi government spokesman denied reports there had been a mass displacement of residents from Sadr City, home to 2 million people and the stronghold of Shi'ite cleric Moqtada al-Sadr's Mehdi Army militia.

They said it was too dangerous to get aid into the eastern Baghdad district, where hundreds of people have been killed in weeks of clashes. Prime Minister Nuri al-Maliki, seeking to impose law and order, launched a crackdown on militias in late March that some analysts believed could trigger an all-out showdown with Sadr.

Dana Graber Ladek, an Iraq specialist at the U.N. International Organisation for Migration (IOM) in Amman, said 500 families fled when U.S. and Iraqi operations began.

"Since then, very few Iraqis have been able to leave due to curfews and insecurity," Ladek said by phone. "We need that corridor opened to allow aid in by U.S. and Iraqi forces, by everyone involved in the conflict."

Ladek said relief was needed urgently. Public distribution of food rations had stopped and food prices were rising.

Water and medical services were running short in the affected areas, especially since a U.S. missile strike near a Sadr City hospital on Saturday damaged a number of ambulances.

"If (the conflict) goes on for very long we risk some more serious consequences like an epidemic of cholera or malnutrition," Ladek said.

Maliki's crackdown was initially launched in the southern Shi'ite city of Basra, where the Mehdi Army put up stiff resistance for a week until Sadr ordered his fighters off the street. Fighting has continued in Baghdad.

The U.S. military said it had killed 17 militants in various battles around Baghdad since Wednesday, mostly using helicopter strikes to respond to attacks on ground forces.

GUNMEN ACCUSED OF BLOCKING AID

Tahseen al-Sheikhli, the government's civilian spokesman for security in Baghdad, accused gunmen of attacking aid convoys trying to reach Sadr City.

"Who is responsible for the deteriorating humanitarian situation in Sadr City? Isn't it the armed groups?" he said. "We have done our best to let food aid reach affected families."

Saeed Haqi, head of the Iraqi Red Crescent, said fewer than 1,000 families had fled Sadr City since the operations began.

Some residents said Iraqi and U.S. security forces had used loudspeakers urging people to leave their homes - perhaps signalling a major offensive was imminent - but Sheikhli and a spokesman from Sadr's office in the slum denied this.

Iraqi security officials gave conflicting accounts, while the U.S. military said claims that its forces were urging residents to leave parts of Sadr City were false.

"It's nonsense," said Lieutenant-Colonel Steven Stover, a spokesman for the U.S. military in Baghdad.

Many residents said they would stay anyway.

"We cannot leave, we cannot abandon our friends," said 30-year-old Ali Qassim.

UNICEF Iraq spokeswoman Claire Hajij said 150,000 people were trapped in the worst-affected quarter of the slum, half of them children. Most stayed in their homes for fear of snipers.

"Service providers have to make a judgement call: is it safe to go to work? Some decide to stay at home and who can blame them?" she told Reuters by telephone from Jordan.

Maliki, himself a Shi'ite, says the crackdown is to disarm militias but Sadr's followers sees it as an attempt to sideline the cleric's mass movement before local elections in October.

The prime minister caught his American backers off guard with his offensive in Basra, but after early military setbacks, it has gone well for his forces. Political leaders across Iraq's sectarian and ethnic divide - apart from the Sadrists, who control 10 percent of seats in parliament - have backed Maliki's campaign.

Sadr, who has a strong following among dispossessed Shi'ites, last month threatened to scrap a truce he imposed on the Mehdi Army in August. A few weeks later he urged his men to observe it, leaving many guessing about his true intentions.