The United Nations on Tuesday warned that a shortage of food and water was worsening the effects of a searing drought in Ethiopia's restive Somali region.

"The overall humanitarian situation in the region has worsened due to progressive shortages of water and food," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement.

"Food and water shortages have reached critical levels in many areas of the region leading to increased rural-urban migration."

The agency, citing aid workers, said five administrative zones in the region, also called Ogaden, are affected because of ongoing military operations against the rebel Ogaden National Liberation Front.

Authorities have denied as exaggerated charges by aid groups that the military operation has hampered delivery of aid to the region, neighbouring lawless Somalia.

In early September, UN humanitarian chief John Holmes called on Ethiopia to grant aid agencies more access in the conflict zone.

Ethiopia's military launched the crackdown last year after the ONLF, a ethnic-based separatist group, attacked a Chinese-run oil venture, killing 77 people.

According to OCHA, some 4.6 million people in Ethiopia need emergency assistance while another eight million require immediate food relief due to the drought.