India's state-run Punjab National Bank has stepped up its controls on the use of SWIFT, a global payments network, following an alleged $1.77 billion fraud, according to memos issued this month and seen by Reuters.

India's second-biggest state lender this month revealed the country's biggest loan fraud, which it alleges was committed by two junior officials at a Mumbai branch, who issued unauthorized "letters of undertaking" (LOU) via SWIFT, for firms linked to billionaire jeweller Nirav Modi and his uncle Mehul Choksi.

The undertakings, which PNB says were issued between 2011 and 2017 with incomplete ledger entries by the two officials, were used by Modi and Choksi's firms to obtain credit from the overseas branches of mostly Indian lenders.