By Tim Cocks and Waleed Ibrahim

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

But 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 district in eastern Baghdad, where weeks of clashes have killed hundreds of people. Prime Minister Nuri al-Maliki, seeking to impose law and order, launched a crackdown on militias in late March.

Dana Graber Ladek, a displacement specialist on Iraq at the U.N. International Organisation for Migration (IOM) in Amman, said around 500 families had fled when U.S. and Iraqi operations against militiamen began.

"Since then, very few Iraqis have been able to leave due to curfews and ... insecurity," Ladek said by telephone.

"We need that corridor open to allow aid in, by U.S. and Iraqi forces ... by everyone involved in the conflict."