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Hundreds of coal miners in Kazakhstan are refusing to return to the surface until their wages are doubled and working conditions improved.

More than 600 miners in the Karaganda region of central Kazakhstan have been underground for four days.

The owner of the mines, a subsidiary of the multinational firm ArcelorMittal, says the strike is illegal and has sued the trade unions that organised it.

But it has offered to negotiate a deal if the miners return immediately.

The strike, which began on 11 December, is a sensitive issue for the Kazakh government.

It is the country's largest workers' protest since the government violently clamped down on striking oil workers in 2011.

Kazakh police opened fire on the oil workers in the western town of Zhanaozen, killing 14.

There are fears of a crackdown in response to the latest strike.

But the interior ministry has denied reports, circulating on social media, that the government has deployed troops and arrested miners.

ArcelorMittal, one of the largest steel and mining companies in the world, says all eight of its mines have stopped producing coal.

In September, the company said it was negotiating a new agreement with the trade unions and planned to sign it before the end of the year.