In a securities filing on Thursday, PNB disclosed that the compliance and monitoring division of the Securities and Exchange Board of India had sent it a warning letter on May 15, stating it had noted delays of 1 to 6 days on the part of the bank in making disclosures related to the fraud - the largest ever in Indian banking history.

PNB, India's second-largest state-run bank, said in February two jewellery groups had defrauded it of more than $2 billion by raising credit from overseas branches of other Indian banks via illegal guarantees issued by rogue PNB staff over several years.

(Reporting by Euan Rocha; Editing by Himani Sarkar)