MANILA, Philippines — Philippine officials on Tuesday recommended the suspension of 20 more mines, mostly nickel producers, for environmental violations, threatening to tighten supply from the world’s top nickel ore exporter.

Environment Secretary Gina Lopez, a staunch environmentalist who has deplored the environmental degradation caused by open-pit mines, made the announcement with her undersecretary, Leo Jasareno.

The Department of Environment and Natural Resources earlier suspended 10 other mines and declared a moratorium on new projects. Three of the 10 mines were included in the initial audit started shortly after President Rodrigo Duterte came to office on June 30. Seven other mines were suspended prior to the audit.

Jasareno said companies named Tuesday will be provided a copy of the audit report and be asked to explain why they committed the violations. If they are eventually closed, the total number of mines shut would rise to 30.

Out of 41 metallic mines audited, only 11 passed the rigorous review, Jasareno said.

“Due to the government’s emphasis on responsible mining, social and environmental standards are becoming more stringent and compliance will remain a major challenge,” said Eufracia Taylor, Philippines analyst at global risk consultancy Verisk Maplecroft.

Investor concerns over nickel production are unlikely to abate, especially as Congress prepares to review mining laws and mulls the possibility of a nickel ore export ban, Taylor said.