The fraudulent transactions were carried by the diamond jeweller Nirav Modi and his companies by acquiring fraudulent letters of undertaking (LoUs) from the lender's Brady House branch at Horniman Circle branch in south Mumbai to secure overseas credit from other lenders.
The agency has a AAA and AA ratings on the various debt instruments of the Delhi based public sector lender which is the second largest amongst the nationalized banks.
"We've placed our ratings on the debt instruments of PNB on 'rating watch with developing implications', following its February 14 disclosure that it has detected some fraudulent and unauthorised transactions at its Brady House branch," the agency said.
The lender had said the fraudulent transactions are contingent in nature and liability arising out of these on the bank shall be decided based on the law and genuineness of underlying transactions.
"We've sought clarity from the PNB management to understand the timeline and the quantum of crystallization of this contingent liability, prospects for recovery, estimated provisioning, potential impact on capitalisation ratios and expectation of additional capital support, amongst others," Crisil said.
The agency, however, said it will remove the ratings from 'watch' and take a final rating action once it has clarity on the matter.
The agency said it continues to factor in the expectation of strong support from the government, established franchise and strong market position in the banking sector, adequate capitalisation and resource profile.