Kazakh media reported they were migrant workers travelling to Russian Federation. Authorities said that only five of 57 people on the bus managed to escape the blaze.
Horrific pictures and video show the inferno from which only five people escaped alive.
The Emergencies Ministry of the former Soviet nation in Central Asia said Thursday's tragedy happened in the northern Aktobe region. "The rest died on the spot".
It is unclear what caused the blaze.
Uzbekistan's foreign ministry said in a statement its embassy staff were en route to the area, roughly midway between the Russian border and the Aral Sea.
Emergency officials have opened a telephone hotline for anxious relatives.
Two of the survivors suffered burns to their hands, emergency services officials said, while others suffered minor injuries.
The cause of the fire has yet to be determined, but investigators believe a short circuit may have caused the bus to ignite.
The bus they were travelling on caught fire and spread quickly.
A photograph taken later showed the blackened vehicle.
He also said the Hungarian-built Ikarus bus was overloaded with passengers.
The Kazakh emergency services ministry said it was travelling from the southern Kazakh town of Shymkent to the Russian city of Samara on the Volga river, a distance of nearly 1,900 kilometres (1200 miles).