SOFIA: Miners at Bulgaria's largest coal producer, state-owned Maritza East Mines, went on strike Sunday after failing to obtain demanded wage increases, miners' union leader Valentin Valchev said.

The start of the strike was also confirmed on state BNT television by company chief executive Evgeny Stoykov.

The night shift of about 1,500 workers refused to go down to their jobs in the three gigantic open-pit mines Sunday evening after management failed to agree to an increase of salaries in accordance with rising ore production.

Over 33 million tonnes of coal ore were extracted from the mines in 2011, surpassing the production target of 27 million tonnes.

Despite receiving certain bonuses, the workers claimed that company management had failed to honour an earlier agreement for more money in accordance with increased production.

Over 3,500 miners staged a one-hour warning strike on Tuesday, stopping coal ore production and shipments to the four electric power plants supplied by the company even if remaining at their jobs.

Maritza East Mines supplies 90 percent of all lignite coal used for electricity production in Bulgaria.

Four coal-fired plants at the Maritza East complex near Radnevo, in central Bulgaria, rely on the mines for their production -- Bulgarian Brikel and Maritza East 2 and US-owned Contour Global Maritza East 3 and AES Galabovo.

They feed a total of 32 percent of all electricity used in the country.

Unions warned that cutting coal supplies to the plants could trigger power outages but Economy and Energy Minister Traicho Traikov ruled out the possibility.

"There are reserves (of power generating capacities) that will be switched on gradually... But it should be clear that this electricity would cost more," Traikov told private bTV television Sunday.

Traikov estimated the strike would cost the company up to 3.0 million leva (1.5 million euros, $2 million) a day and warned the miners that "when the strike is over it may turn out they have brought the company to bankruptcy."

Stoykov meanwhile called the strike "illegal".

He reminded miners that they received a 15-percent rise in salaries last year and ruled out another increase as "unthinkable at times of crisis".

Payment rows have been frequent over the past year at Maritza East Mines, which employs about 7,000 miners working at two 12-hour shifts for an average monthly salary of about 1,500 leva.