The International Federation of the Red Cross and Red Crescent Societies (IFRC) says there is growing concern among aid groups of potential disease outbreaks.

"Already, some cholera cases have been reported in Beira along with an increasing number of malaria infections among people trapped by the flooding," the federation said in a statement on Friday.

Cyclone Idai battered Beira, a port city of 500,000 people, with strong winds and torrential rains last week, before moving inland to neighbouring Zimbabwe, where it flattened homes and flooded communities, and Malawi.

The storm killed 242 people in Mozambique and 259 in Zimbabwe, and numbers were expected to rise, relief agencies said. In Malawi, 56 people died in heavy rains before the onset of Idai.

Cholera is spread by faeces in sewage-contaminated water or food, and outbreaks can develop quickly in a humanitarian crisis where sanitation systems are disrupted. It can kill within hours if left untreated.

As survivors gathered in informal camps and health officials warned of the danger of cholera and other diseases, UNICEF executive director Henrietta Fore said the situation on the ground was critical, with no electricity or running water.

"Hundreds of thousands of children need immediate help," she said, estimating 1.7 million people were affected by the storm.

Around 45km west of Beira, in the village of Guara Guara, the government set up a makeshift camp for people rescued nearby, with little water and no toilets.

As for many such camps, progress was slow as aid had to be delivered by helicopter.

Sixty-year-old Esther Zinge, from near the town of Buzi, said help was coming "very slowly" and had to be given to children first.

"The conditions are terrible, and more people keep coming," she said.

United Nations Secretary-General Antonio Guterres said he was saddened by the "heart-wrenching images of human suffering" and urged the world to step up support for the relief effort.

In Zimbabwe's Coppa Rusitu Valley, a township in Chimanimani near the Mozambican border, hundreds of homes were flattened by large rocks and mudslides from a nearby mountain, burying some residents at night when most were sleeping.

Some survivors have taken refuge at churches and centres offering temporary shelter as they deal with the trauma of their losses while private citizens, international aid agencies and the government rushed humanitarian aid to affected areas.

Zimbabwean Energy Minister Joram Gumbo said the docking terminals at Beira port had been damaged and that Zimbabwe had 62 days' supply of petrol and 32 days for diesel, with long queues in the capital.

Australian Associated Press