SARAJEVO — 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.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.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. 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."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.Thousands of volunteers have responded to government's appeal to help build up flood defenses along the Sava. Bused in from all over the country, the volunteers spent the night building sandbag barricades with soldiers and emergency crews.Both Serbia and Bosnia have appealed for international help. In Bosnia, many lost homes they had only just rebuilt after the 1992-95 war, which claimed 100,000 lives and devastated the impoverished country. Scores of landslides hit as sodden hills gave way, also presenting a huge problem.Residents in both countries have mobilized through Facebook or other social media, collecting tons of food, blankets and clothing for the crisis-hit areas.