Maputo - Floods in southern Africa have killed about 45 people in a growing humanitarian crisis that has engulfed the region and brought renewed appeals for Western financial help.

Heavy rains have caused rivers in Zambia, Zimbabwe, Mozambique and Malawi to burst, killing three people in Malawi since Friday and forcing hundreds of others to flee their homes.

Heavy downpours are common in southern Africa during the rainy season, which runs generally from November to April, but the relentless rain is unusual and has caught officials off guard.

In Zimbabwe, state media have reported 27 people have been killed by floods but relief officials have not confirmed the figure. Some victims in the region were killed by crocodiles.

Twelve people have been killed and tens of thousands displaced in Mozambique in the last two weeks, the national relief agency says. UN agencies say three have been killed in Zambia.

In Malawi, floods swept away livestock and inundated agricultural land.

"Crops and livestock have either been destroyed or displaced and people seeking refuge are at risk of drowning as most rivers are swollen," Lowford Palani, the acting commissioner in Malawi's flooded Chikwawa district, told Reuters on Thursday.

Palani said more than 200 people in 24 villages in the southern district had been displaced since last week when heavy rains caused the Shire and other rivers to overflow.

The flooding in Malawi came as authorities in neighbouring countries warned that the crisis could devastate farms and destroy roads and bridges.

Mozambique braced for more heavy flooding on Thursday as water in one of its largest rivers rose above alert levels, relief authorities said.

The Limpopo River was receiving heavy volumes of waters from Zimbabwe, said the director of the National Institute of Disaster Management, Paulo Zucula.

"We had water levels going up two metres (6 feet) high in just 24-hours at Combomune, (the entrance into Mozambique) giving us a strong indication of heavy flooding in a few days", Zucula told Reuters.

"We are re-evacuating people from areas which we had initially declared safe but now are highly dangerous...," he said, adding that the floods have killed 12 people.

Zambian President Levy Mwanawasa launched a fresh appeal for US$13 million in aid from Western donors to help his government cope. It came after flooding cut off a key trade route with Malawi and Mozambique.

"Normally it is not right to appeal for support at occasions of this nature, but I am concerned with the damage being caused to key infrastructure, hence my appeal to donors," Mwanawasa told state-owned television late on Wednesday.

Concerns are especially high in Zimbabwe, which has struggled to feed itself amid a deep economic slide that has been marked by chronic shortages of food and fuel, rising poverty and inflation over 8,000 percent.

Zimbabwe's meteorological services department dashed hopes of a break in the weather when it said on Wednesday that "significant rains" and heavy storms in the coming days could worsen the flooding in the northern part of the nation.

(Additional reporting by Shapi Shacinda in Lusaka, Frank Phiri in Blantyre and Nelson Banya in Harare; Editing by Giles Elgood)