Emergency officials in Kazakhstan have reported that at least 52 people were killed on January 18 after a bus burst into flames on the highway running north to south from Uzbekistan to Russia.

Tengri News cited officials as saying the accident occurred in the middle of the morning in the Yrgyz district of Kazakhstan’s northwestern Aktobe region.

“Ten kilometers from the village Kalybai, while the vehicle was in motion, an Ikarus passenger bus burst into flames. There were 55 passengers and two drivers in the bus,” Emergency Services Committee official Ruslan Imankulov told Tengri News.

The five surviving passengers are receiving treatment for their injuries, Imankulov said. Nur.kz news website cited officials as saying the passengers killed were from Uzbekistan.

It is not yet known what caused the blaze.

Buses are a popular alternative form of transportation to planes for migrant laborers traveling to and from Russia, for a number of reasons. Sometimes it is the unavailability of affordable air tickets, but as Mansur Mirovalev documented in a long-read report for the Associated Press in 2009, entry into Russia through overland borders is often a preferred option for those seeking to avoid detection by migration officials.

“Each year, hundreds of Uzbeks without permits are deported and barred from entering Russia for five years. They can't get through passport controls at airports or railway stations, so they get on the bus,” Mirovalev wrote.