The Mozambican authorities raced to evacuate tens of thousands of residents from the flood-drenched south of the country on Wednesday before a fresh swell of water hits.

The first phase of the emergency operation kicked into gear, with teams using 10 rescue boats to move 30,000 people from the worst-hit areas around the district of Chokwe.

"The scenario is critical. The population is being evacuated where necessary," said Rute Nhamucho of Mozambique's national water directorate.

Up to 55,000 people are thought to be in the danger zone, although some may be able to move of their own accord.

"The immediate plans are for 6,000 families -- approximately 30,000 people," to be moved, Hanoch Barlevi, an emergency specialist with UNICEF, told AFP.

Three hospitals were also evacuated, forcing patients to stay in tents.

Flooding is expected to intensify in the coming days as the swollen Limpopo River dumps a mass of rains from neighbouring South Africa and Zimbabwe on Chokwe.

"It is difficult to say when, but they are expecting it to get worse. The big wave is to come," Barlevi said.

As much as 10,000 cubic metres per second was flowing in the river.

The sheer volume forced the authorities to open dams, preventing a potentially disastrous failure, but spilling water into the surrounding areas.

But the authorities face a race against time and a host of logistical difficulties to get people to safety.

Floods have cut access to several areas and communications are difficult.

Emergency teams had to bring tents and food to the border district of Pafuri through neighbouring South Africa, which itself has been hit.

In the neighbouring South African province of Limpopo 10 people have been killed and four are still missing.

About 700 houses remain inaccessible to the rest of the region, according to a government spokeswoman.

Local officials, some of whom are pushing to declare the region as a disaster zone, voiced concerns on the possible impact the flood will have on farmers.

"From what we've seen, it will be devastating," said Dieketseng Diale, a local government official. "It's a bit too early, but damage will probably be in the millions (rand)."

Back in Mozambique there are fears that popular tourist hub of Xai Xai on the southern coast may be affected.

"It is critical to move people away from the river so they do not stay in low-lying zones. A big volume of water like that can come as a surprise," said Nhamucho.