China said Thursday it would expand export quotas for rare earths to include iron alloys containing the elements, further tightening shipments of the minerals used in a variety of high-tech industries.

From Friday, iron alloys containing more than 10 percent of rare earths by weight will fall under the export quota system, the commerce ministry said in a statement.

The ministry said the move would "more effectively protect the exhaustible materials and the environment".

China has previously imposed export quotas for pure rare earths only -- 17 elements critical to manufacturing everything from iPods to low-emission cars, wind turbines and missiles.

The decision to include alloys into the existing quotas is expected to close loopholes by which exporters can sidestep current regulations, thereby further reducing the amount of rare earth elements in the country's exports.

China produces more than 95 percent of the world's rare earths.

Beijing, keen to burnish its green credentials and tighten its grip over the highly sought-after metals, has started cleaning up the industry by closing illegal mines, setting tougher environmental standards and restricting exports.

The government has cut rare earth exports for the first half of 2011 by 35 percent compared to a year earlier, having slashed the quota by 72 percent for the second half of last year.

The moves have prompted complaints from foreign high-tech producers while the United States and Australia have responded by developing or reopening mines shuttered when cheaper Chinese supplies became available.