MANILA (Reuters) - The Philippines will close down over 20 mines, mostly nickel producers that account for about half of output in the world's biggest nickel ore supplier, as a government campaign to fight environmental degradation deepens.

Manila is also suspending operations at six other mines, including the country's top gold miner.

Environment and Natural Resources Secretary Regina Lopez ordered the closure of 21 mines and the suspension of several others, including a gold mine operated by Australia's Oceanagold Corp, for causing environmental destruction.

Shares of Oceanagold fell more than 14 per cent.

"Why is mining more important than people's lives?" Lopez told a media briefing.

Lopez, a staunch environmentalist, said several of the mining operations that were shut were in functional watersheds.

"My issue here is not about mining, my issue here is social justice," she said, after showing footage of environmental damage caused by mining in the South-east Asian nation.

Lopez said the nickel mines ordered to shut account for about 50 per cent of the country's annual output. These mines, along with others, can appeal their case to President Rodrigo Duterte.

Duterte has backed Lopez's mining audit, warning shortly after taking office last June that the Philippines could survive without a mining industry.

Some of the mines that have been ordered to close had their production suspended last year by the government, leading to a spike in nickel prices.

Three-month nickel on the London Metal Exchange fell 0.4 per cent to US$10,210 a tonne by 1.33pm Singapore time, with a holiday in China dampening trading.

Oceanagold, in a statement, said it has not received any official suspension order from Manila's Department of Environment and Natural Resources. "There is no legal basis for any proposed suspension," the miner said.