The bus, which was mostly carrying high school students, was hit by a truck during rainy weather and ended up in a ditch.
A crash between a truck and a passenger bus killed 12 people and injured 17 in western Slovakia on Wednesday, in the central European country's deadliest road accident in a decade, Slovak fire and rescue services said.
The accident occurred in rainy weather on a winding two-lane road near the city of Nitra, 90 km east of the capital Bratislava.
No details were immediately available on the casualties, which company operated the bus or what may have caused the accident.
Commercial broadcaster Markiza reported that the bus was carrying mostly high school students, and it was struck on its side by the truck before it crashed into a ditch alongside the road.
Several people remained trapped in the bus after the crash, police said.
Prime Minister Peter Pellegrini said in a statement he would end an official visit to Cyprus one day earlier to return to Slovakia and visit the site of the accident.
The accident was the deadliest in the country since 2009, when a train crashed into a bus and killed 12 people.