As a new cyclone advanced on flood-ravaged Mozambique, the United States mobilised rescue and disaster assistance workers, mindful of hundreds of deaths caused by floods earlier this year.

The South African National Defence Force (SANDF), which in the earlier disaster played a key role in airlifting thousands of trapped flood victims to safety, was on alert to relocate aircraft to areas under threat.

Disaster management authorities say the new storm may affect up to 400 000 people in the previously unscathed northern provinces of Nampula and Zambezia.

The weather bureau in Pretoria on Wednesday said Cyclone Hudah, which has now been downgraded to an intense tropical storm, was moving at about five knots, or 10km/h, down the Mozambican coast.

The storm, about 250km to the north of Beira, was causing gale-force winds of 30 to 40 knots, which would almost certainly destroy any houses made of flimsy material that lay in its path, said a forecaster.

A high-pressure system was preventing Hudah from moving inland, increasing the possibility that it would intensify into a cyclone as it sucked warm, moist air from the Mozambican channel.

The forecaster said the eye of the storm was clearly visible on satellite photographs and added that the storm was not expected to "land" on Thursday.

Cyclone Hudah has already killed at least 13 people on the island of Madagascar.

Most Western countries have recalled their personnel from Mozambique, although Australia still maintains its contingent which is manning a hospital and water purification plant in Chibutu, 150km north of Maputo.

The SANDF has maintained a presence of about 50 soldiers and military personnel as well as a number of aircraft, used to ferry food and medical supplies to flood victims.