A car drives by a flooded area after Cyclone Idai in Beira, Mozambique. Picture: CARE International/Josh Estey via Reuters

Beira, Mozambique - Rescue workers in Mozambique fear that thousands more may have fallen victim to floods which have left a 125-kilometre lake after the African country was hit by cyclone Idai.

The storm made landfall near Beira on March 14, leading to storm surges and massive flooding, as well as a massive body of water in an area where hundreds of thousands of people live.

In the Buzi region near the severely affected city of Beira, a lake 125 kilometres long and 11 metres deep has formed, Pedro Matos, emergency aid coordinator of the World Food Programme (WFP) in Mozambique said on Thursday.

"Either they were able to flee or there are very large numbers of victims." At the moment helpers were mainly engaged in rescue operations, he added.

In Beira's hinterland, river levels continue to rise due to prolonged rainfall. Thousands of people are believed to have sought refuge on rooftops and in treetops, the UN said.

People affected by Cyclone Idai sit in a boat in Beira. Picture: CARE International/Josh Estey via Reuters

In the inland province of Manica, more than 100,000 people are cut off from outside help because transport routes are destroyed, WFP spokesman Herve Verhoosel said in Geneva.

"The situation is likely to deteriorate and the number of people affected is expected to increase," Verhoosel told reporters, pointing to the heavy rains that were expected to continue Thursday.

Cyclone Idai, a Category 4 storm, could be one of the biggest cyclone disasters south of the equator, according to the World Meteorological Organization (WMO), and has brought devastation to several countries in the region.

People stand on a road damaged by Cyclone Idai in Beira. Picture: CARE International/Josh Estey via Reuters

According to the authorities in the southeastern African nation, at least 200 people have been reported dead so far. On Monday, the government warned that it feared up to 1 000 fatalities.

Aid workers expect up to 400 000 people to be temporarily homeless. After the cyclone hit Mozambique last week, it headed towards Zimbabwe.

According to WFP's preliminary assessment, 200 000 people in Zimbabwe will need urgent food assistance for three months.

Verhoosel reported that heavy rains continue to cause massive destruction in areas near the border with Mozambique.

The Marowanyati dam has overflown, raising river levels and putting populations at risk.

In nearby Malawi, 920,000 people have been affected by the floods that already started in early March.

"Cyclone Idai had limited impact in Malawi," Verhoosel said. "The number of people affected is expected to decrease as people return to their homes."