While several reports indicated five deaths and two missing workers, Turkish mining officials told the Associated Press on Monday afternoon that eight had perished at the Kozlu coal mine in the Zonguldak province.
Agency head Burhan Inan did not indicate the specific time of the leak but said five of the bodies had been recovered by midday.
Another six were evacuated unharmed and one miner escaped with injuries.
Inan said the group had been working at the time to open up a space in a tunnel to reach a deposit when they were overtaken by a sudden methane discharge.
Local media outlets said the group was working about 630m underground.
Russian news outlet RAI Novosti reported that seven workers were hospitalized Monday for methane gas poisoning.
Turkey has been working to improve safety at its operations, even idling dangerous and unlicensed mines.
The country’s most deadly mine disaster ever also occurred in the province when about 270 workers were killed in a gas explosion there in 1992.