India's Adani Group has appointed accountancy firm Grant Thornton for independent audits of some of its companies in a bid to discredit claims by short-seller Hindenburg Research that have battered its stocks and bonds, two people familiar with the matter said on Monday.

The appointment marks the first major effort by Adani Group to defend itself in the wake of a Jan. 24 report by Hindenburg that accused it of improper use of offshore tax havens and stock manipulation.

The conglomerate, led by billionaire Gautam Adani, has strongly denied the allegations but investors remain concerned. Shares in the group's seven listed subsidiaries have cumulatively lost about $120 billion in market value in the last three weeks.

Adani Group said last week it was considering independent evaluation of issues relating to legal compliance, related party transactions and internal controls following the Hindenburg report. The appointment of Grant Thornton is reported here for the first time.

Grant Thornton has been hired to conduct independent audits of some Adani Group companies, said the sources, declining to be named as the appointment is confidential.

One of the sources added that Grant Thornton would look at whether related-party transactions at the Adani Group complied with corporate governance standards.

Grant Thornton and Adani Group did not immediately respond to a request for comment.

Adani Group sought to reassure investors on Monday, saying it had strong cashflows, its business plans were fully funded and that it was "confident in the continued ability of our portfolio to deliver superior returns to shareholders."

But regulatory pressure has been increasing. India's market regulator confirmed on Monday it was investigating the report by Hindenburg, as well as market activity immediately before and after the report was published.

The U.S. short-seller's report said it had identified numerous "undisclosed related party transactions" by both listed and private Adani companies, alleging it was in violation of Indian disclosure laws.

In its rebuttal, Adani had said "all related party transactions are at arm's length, properly disclosed and reviewed/audited by statutory independent auditors." (Reporting by Aditya Kalra in New Delhi and Greg Roumeliotis in New York; Editing by Richard Chang)