* Worst rains and floods for at least 120 years hit region
* Around 135,000 without electricity in Serbia
* Five killed and thousands evacuated from homes (Adds Polish details, Serbian evacuation, latest death toll, details from Bosnia)
By Fedja Grulovic
OBRENOVAC, Serbia, May 16 (Reuters) - Boats sailed through the streets of a Serbian town on Friday on a mission to rescue people trapped by rising waters as the worst floods ever recorded swept Serbia and Bosnia.
Some residents of Obrenovac, 30 km (20 miles) 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 this week. Thousands have been evacuated from homes in central and western areas of Serbia and in neighbouring 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.
"This is a catastrophe. Nature has never been so cruel to us," Serbian Energy Minister Aleksandar Antic said.
The heaviest rains since records began almost 120 years ago have hit Serbia and Bosnia this 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.
In Bosnia, an army helicopter evacuated a pregnant woman in labour from her flooded house in the town of Doboj and took her to hospital in the central town of Zenica. Low clouds and fog were making rescue efforts difficult.
In the village of Topcic Polje, near Zenica, a landslide devoured dozens of houses and water flooded the main road.
Villagers fled on foot along railway tracks with bags and babies in their arms.
"The people are walking to Zenica, all the roads are jammed, we are stuck here and there is no help from anyone," villager Asim Skopljak told Reuters by telephone from Topcic Polje. "There is no electricity and no drinking water."
Surges of high water were expected to reach the major rivers Sava and Danube later in the day and over the weekend, threatening thousands more people and roads, meteorologists in Serbia said.
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.
Poland also saw less severe flooding after the torrential rains across eastern Europe.
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.
Some local trains were out of service, Poland's state railways PKP said. News agency PAP reported that 21,000 people in the south had no electricity. (Additional reporting by Ivana Sekularac, Daria Sito-Sucic, Maja Zuvela, Dado Ruvic and Wojciech Zurawski in Poland; editing by Zoran Radosavljevic and Andrew Roche)