MELBOURNE (Reuters) - Outgoing Rio Tinto Chief Executive Jean-Sébastien Jacques said on Friday the miner was committed to reform after a lawmaker accused the industry of incremental cultural genocide at an Australian inquiry into the destruction of an ancient cave.

Speaking for a second time at a parliamentary inquiry into how the world's biggest iron ore miner destroyed a culturally significant 46,000-year-old rockshelter in Western Australia, Jacques repeated an apology and vowed Rio would improve its efforts.

"It should have never happened," Jacques told the inquiry into the destruction of the rockshelters at Juukan Gorge as part of an iron ore mine expansion in May, which caused deep distress to the Puutu Kunti Kurrama and Pinikura (PKKP) people.

Rio's CEO, corporate relations chief Simone Niven and head of iron ore Chris Salisbury all faced the inquiry after handing in their resignations last month, bowing to a shareholder outcry over the destruction of the rockshelters and what was seen as the miner's inadequate initial response.

"Anyone that has been objectively listening to this inquiry ... would have to draw the conclusion that this is a form of incremental genocide," Senator Pat Dodson, an Aboriginal man, told Rio at the inquiry.

Dodson referred to confidentiality clauses in mining agreements that restrict traditional owners from public comment about their heritage concerns, and clauses that restrict their ability to use broader Australian laws for site protection.

He also said that the mining industry had taken advantage of "weak laws" in Western Australia that allowed miners to apply to destroy sites, but that did not give a traditional owners a right to appeal. The heritage laws are undergoing reform.

"You are an international company. You have destroyed significant heritage for humanity ... Saying sorry is one very important matter. And it seems to me that your future reputation will swing very much, as will other companies', on how they behave to First Nations peoples," Dodson added.

Rio has taken steps to reform its cultural heritage management practices, including changing reporting lines and responsibilities, and putting a 57 square km (22 square mile) mining moratorium on the affected lands of the PKKP.

"We fully agree and that is why we are absolutely committed to change, to reform. We have started, we have already taken action, there is more work to be carried out for all the reasons you have just mentioned," said Jacques, who is due to step down by the end of March.

Rio pledged to modernise its agreements and create stronger heritage protection in a letter to the 12 Aboriginal groups it has agreements with in Pilbara region, signed by acting head of iron ore Ivan Vella.

In the letter, supplied by Rio, it said it would not enforce confidentiality clauses restricting groups from raising cultural heritage concerns, or those that would impede applications for legal protection of any cultural heritage sites.

The incident has sparked a review by Australian miners, including peers BHP Group and Fortescue, of their cultural heritage management practices, and led large institutional investors to focus on First Nations heritage management.

(Reporting by Melanie Burton; editing by Richard Pullin, Kirsten Donovan)

By Melanie Burton