Beira (Mozambique) (AFP) - The death toll from a powerful cyclone that pummelled swathes of southern African countries, flooding thousands of square kilometres, on Saturday surged past 600 as diseases stalked tens of thousands of survivors.

At least 417 people have died in Mozambique, according to government, bringing to 676 the total deaths when combined with those from neighbouring Zimbabwe.

Cyclone Idai smashed into the coast of central Mozambique on Friday last week, unleashing hurricane-force winds and rains that flooded the hinterland and drenched eastern Zimbabwe leaving a trail of destruction.

Relief and rescue efforts entered a second week in the central parts of the impoverished coountry.

The UN, warning of more suffering, stepped up calls for help in Mozambique as aid agencies struggle to assist tens of thousands of people battered by one of southern Africa's most powerful cyclones.

- 'Situation will get worse' -

A week after the storm lashed Mozambique with winds of nearly 200 kilometres (120 miles) per hour, survivors are struggling in desperate conditions -- some still trapped on roof tops and those saved needing food and facing the risk of outbreaks of disease such as cholera.

"The situation will get worse before it gets better," UNICEF Executive Director Henrietta Fore said Saturday.

"Aid agencies are barely beginning to see the scale of the damage," she adding that "entire villages have been submerged, buildings have been flattened, and schools and health care centers have been destroyed".

The storm wreaked an area about 3,000 square kilometres of land.

The World Food Programme late Friday night declared the flood crisis a level three emergency, putting it on a par with crises in Yemen, Syria and South Sudan.

"The designation will accelerate the massive operational scale-up now underway to assist victims of last week’s Category 4 cyclone and subsequent large-scale flooding that claimed countless lives and displaced at least 600,000 people," said WFP spokesman Herve Verhoosel.

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"Now that the world is beginning to grasp the scale of devastation and despair," he said.

More than two million people have been affected in Mozambique, Zimbabwe and in Malawi where the storm started as a tropical depression causing flooding which killed 60 and displaced nearly a million people.

Hundreds are still missing in Mozambique and Zimbabwe.

- 'Like Yemen, Syria' -

Humanitarian agencies are racing against the clock to help people, many of whom have not had a meal in days.

Poor sanitary conditions mean disease is now a real concern.

"Already, some cholera cases have been reported in (the port city of) Beira along with an increasing number of malaria infections among people trapped by the flooding," the International Federation of the Red Cross said in a statement.

Even some of those rescued and in shelters are in overcrowded conditions and poor hygiene places them at risk. UNICEF estimates at least a million children have been affected by the storm.

Aid group Doctors Without Borders said people were also at risk of respiratory infections such as pneumonia.

In Buzi, a small town that has reportedly "disappeared" under water, an AFP photographer saw people sleeping outside, under the rain and many coughing.

"There are lot of dead (people). We can't even count," said Otelea Jose, arriving by boat in Beira.

"We are living an unprecedented natural disaster. A disaster that only matches major disasters," said Lands and Environment Minister Celso Correia.

Districts west of Beira resemble an inland lake, and thousands of people are still trapped on roof tops.

- 'Unprecedented natural disaster' -

Although water levels are slowly going down, according to the government,the UN humanitarian agency warned of fresh flooding if more rains fall.

"The Zambezi (river) is essentially in flood currently, it's very high .. there is lot of rain in the highlands, the more that flows down to the Zambezi the more likely that is going to break its banks and we will have a second flooding emergency," said Sebastian Stampa, an OCHA coordinator.

Nearly 90,000 people are already in shelters in central Mozambique.

In Beira, businessman Ibraimo Masquine counted his blessings. "I can't believe I'm here. I was scared for my life," he said clearing debris from his factory, but he also said there was "no clean water to drink, no food".