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.”
The storm swept across Mozambique on Thursday night before turning inland into neighboring Malawi and Zimbabwe. It is believed to have affected more than 1.5 million people in the three nations.
The countries, among the world’s poorest, have limited capacity to respond to the disaster, and officials called for outside help. Relief officials warned that delays in reaching survivors could lead to an outbreak of illnesses, including cholera and malaria.
“This cyclone has come at a very huge human cost,” Nick Mangwana, a spokesman for the Zimbabwean government, said on Twitter. “We certainly need a regional approach to these effects of global warming. These problems are transnational.”
In Mozambique, several aid agencies focused their efforts on Beira, a port city of half a million people that was all but destroyed by the storm. A cargo plane loaded with emergency supplies was able to land at the airport there, but aid groups struggled to distribute the goods to affected areas.
The main highway into the city remained impassable, while secondary roads were being cleared of fallen trees and other debris, said Saul Butters, CARE’s assistant director for Mozambique.
“We have 13 articulated trucks that are unable to get into the city,” Mr. Butters said, adding that the agency was now organizing airlifts by helicopter.
Until isolated communities are reached, relief workers said, it is impossible to estimate the number of casualties. The official death toll in Mozambique was 84, though President Filipe Nyusi said on Monday that it could climb to more than 1,000.
Beira, which is Mozambique’s fourth-largest city and faces the Indian Ocean, remained without electricity and telephone service on Tuesday.
After hitting Beira on Thursday night, the cyclone moved the next day into Chimanimani, a rural district in Zimbabwe near the border with Mozambique.
Officials at Doctors Without Borders said they had been unable to reach Chimanimani because of destroyed roads and bridges, but the group was providing services at a center outside the area.
On Tuesday in Chimanimani, burials were held for some of the victims of the storm.
Absalom Makanga, a leader of the Salvation Church in Chimanimani, said by phone that 40 victims in a low-income area called Ngangu had been buried.
“Our resident pastor officiated at the burials because we could not get there because of the poor state of the roads destroyed by the cyclone,” Mr. Makanga said.
Rutendo Rukavi, a volunteer who was helping the flood victims in Chimanimani, said that she had witnessed makeshift burials for the dead, including for an 83-year-old man.
“It was sad,” Ms. Rukavi said. “People were just being buried in the midst of the debris that remained of their homes.”
In Malawi, heavy rains have caused severe flooding in 14 southern districts near the Mozambican border, including Chikwawa and Nsanje.
In Chikwawa, the worst-affected area, nine people have died and more than 54,000 people have been displaced.
Lusizi Mshani, district commissioner for Chikwawa, said by phone that the displaced were staying in severely overcrowded government-run camps, each holding about 5,000 people.
Mr. Mshani said that most people affected by the floods had now been moved to the camps.
“The water level has gone down, and people in the lower areas have been moved to higher ground,” he said. But, he said, the displaced were now in desperate need of new shelter, food, clean water and medicine.