News reports and officials say a passenger bus in Turkey has veered off a snow-covered highway and tipped on its side, killing 21 people and injuring some 20 others.

Gov. Orhan Duzgun said bus travelling from Istanbul to the eastern province of Mus crashed in the central province of Kayseri early on Thursday.

The cause of the accident was under investigation and it was not known if foggy weather and snow were a factor.

The private Dogan news agency said police were also investigating the possibility that the driver may have fallen asleep.

Traffic accident rates are high in Turkey where driving is poor and road safety rules are inadequately enforced.