BELGRADE, Serbia — Tens of thousands fled their homes Saturday in Bosnia and Serbia, evacuated by boat or helicopter as rising waters surged into villages and towns. Authorities said the record flooding killed at least 20 people and the death toll could rise further.

Meteorologists say the flooding is the worst since records began 120 years ago and is due to a three-month amount of rain that fell on the region in just three days. Goran Mihajlovic from Serbia’s Weather Center said that such a rainfall happens once in 100 years.

In the eastern Bosnian town of Bijeljina, some 10,000 people were being evacuated Saturday after the rain-swollen Sava River pushed through flood defenses. Mayor Mico Micic appealed for help, saying “we need everything, we are under water.”

Officials in Bosnia say 12 people died and more bodies could emerge as water recedes from the dozens of cities flooded in the past three days. In some places, floodwaters had reached the second floor of people’s homes and they had to be rescued by helicopter from their roofs.

In Serbia, which saw eight deaths, emergency crews and soldiers were using boats and helicopters to rescue thousands trapped in the town of Obrenovac, near Belgrade. Officials said more than 15,000 people have been evacuated so far from the flood-hit regions, most of them finding shelter in schools and sports halls.

The flooding in Obrenovac is threatening the Nikola Tesla power plant, Serbia’s biggest. Plant capacity had already been cut after a nearby coal mine was flooded.