MAPUTO, Mozambique — Rescue workers struggled Tuesday to reach areas devastated by a huge cyclone in Mozambique, as heavy rains swelled rivers and further isolated flooded communities in what aid agencies called the worst natural disaster in southern Africa in two decades.

In central Mozambique, the area hit hardest by Cyclone Idai, overflowing rivers created “an inland ocean” where countless people were still marooned, a United Nations official said.

Rescue workers reported seeing people on rooftops and in trees days after the storm struck. In areas near the rivers, homes were submerged, with water rising near the tops of telephone poles.

“We took an aerial survey, and as far as the eye can see, there was flooding, and deep as well,” said Jamie LeSueur, who was leading rescue efforts in central Mozambique for the International Federation of Red Cross and Red Crescent Societies. “What we are now facing is large-scale flooding on top of an area already devastated by the cyclone.”