Anglo-Australian miner Rio Tinto has yet to be presented with any evidence to support the detention of four of its China-based staff on suspicions of stealing state secrets, the firm said on Tuesday.

Chinese authorities detained Rio Tinto's top iron-ore salesman in China, Australian Stern Hu, and three Chinese colleagues a month ago, accusing them of spying on Chinese steel mills, major buyers of the company's iron ore.

The four have yet to be officially charged.

We are still not aware of any evidence that would support their detention, Rio Tinto's iron ore division chief, Sam Walsh, said in a statement.

We continue to be concerned for the health and welfare of our three other employees detained at the same time as Stern Hu, he said, noting that the Australian government had informed the company that Hu was well.

(Reporting by Mark Bendeich; Editing by Jonathan Standing)