People in parts of Mozambique, Zimbabwe and Malawi are now picking through the wreckage left behind by Cyclone Idai, a category four storm that brought weeks of rain, floods and high winds to a swath of southeastern Africa.

Mozambique was hit especially hard. About 450 people died in Mozambique, 260 in Zimbabwe and 55 in Malawi, according to preliminary estimates from officials. Rescue workers say that thousands more in coastal areas of Mozambique may have been swept out to sea as the floodwaters rose. Their bodies may never be found.

At least three million people were affected by the devastation, said Antonio Guterres, the Secretary General of the United Nations. He called it “one of the worst weather-related catastrophes in the history of Africa,” and urged the international community to send relief and funds quickly. He called Cyclone Idai an “uncommonly fierce and prolonged storm” that is “yet another alarm bell about the dangers of climate change, especially in vulnerable, at-risk countries.”