Some residents of Obrenovac, 30 km southwest of the capital Belgrade, were stranded on the roofs of their homes, calling for help. Prime Minister Aleksandar Vucic said all 25,000 citizens would have to be evacuated.
At least five people have died in the unfolding disaster last week. Thousands have been evacuated from homes in central and western areas of Serbia and in neighboring Bosnia.
Around 135,000 households were without power across Serbia and the government approved emergency electricity imports. Another 65,000 were without electricity in Bosnia.
The heaviest rains since records began almost 120 years ago have hit Serbia and Bosnia last week. Three people, including a rescue worker, drowned in Serbia. At least two villagers died in landslides near the northern Bosnian town of Bijeljina.
The deluge has made many hillsides unstable in the mountainous region. Several people were injured when houses were destroyed by a landslide on the edge of the Bosnian capital Sarajevo.
Bosnia's Centre for the Removal of Landmines warned that mines laid during the country's 1992-95 war could be moved by floods and landslides.
Bosnia's central government appealed on Friday for international help. Russian emergency teams with rescue boats arrived in Serbia on Friday and were heading for Obrenovac to help with the operation there.
Israel, Turkey, Slovenia, Croatia, Austria and Luxembourg have pledged to send aid including expert teams, water pumps, helicopters and rescue boats.
The United States said it would send 13 motor boats needed for evacuations in Bosnia.
The road to the Slovak border crossing in the southern village of Leluchow was closed and dozens of houses on the bank of the river Poprad were inundated.