The CBI will extend all necessary help to the United Kingdom authorities for extradition of fugitive diamantaire Nirav Modi, wanted in Rs 13,000 crore fraud in Punjab National Bank in collusion with his uncle Mehul Choksi, the agency said Saturday.

The CBI is awaiting a response on its extradition request which was sent to the United Kingdom through the External Affairs Ministry in August last year after the confirmation from London that Modi was in their country, agency spokesperson Nitin Wakankar said.

The United Kingdom has responded to the Red Corner Notice issued by the agency against Modi in June last year, he said.

"We are willing to extend all help to the United Kingdom through External Affairs Ministry in ensuring extradition of Nirav Modi," Wakankar said.

Modi fled the country after allegedly siphoning off about Rs 13,000 crore from Punjab National Bank using Letters of Undertaking in collusion with his uncle Mehul Choski.

Forty-eight-year-old Modi was spotted in a tony neighbourhood of London by British newspaper The Telegraph.

He refused to comment to any of posers put forth by The Telegraph correspondent.

The CBI registered an FIR against him on January 31 on the basis of a complaint against him and his uncle Choksi from the bank. It was followed by another FIR by the agency against him.

Modi's brother and wife were also named as accused in the FIR.

His wife Ami, a US citizen, brother Nishal, a Belgian, and uncle Choksi, Gitanjali Group's promoter, had also fled the country in the first week of January.

The case pertains to allegedly cheating the state-run Punjab National Bank (PNB) through fraudulent issuance of Letters of Undertaking (LoUs) and Foreign Letters of Credit (FLCs).

The agency has charge-sheeted both Nirav Modi and Choksi separately in the scam. 

It has now approached the Interpol for a Red Corner Notice aimed at bringing Nirav Modi back for facing trial in the cases against them, the sources said.

The CBI, in its charge sheets filed on May 14, had alleged that Nirav Modi, through his companies, siphoned off funds to the tune of Rs 6,498.20 crore using fraudulent LoUs issued from PNB's Brady House branch in Mumbai.  Choksi allegedly swindled Rs 7,080.86 crore, making it possibly the biggest banking scam in the country, it alleged.

It is alleged that Nirav Modi and Choksi through their companies availed credit from overseas branches of Indian banks using fraudulent guarantees of the PNB given through LoUs and letters of credit which were not repaid bringing the liability on the state-run bank, the officials said.

An LoU is a guarantee given by an issuing bank to Indian banks having branches abroad to grant short-term credit to the applicant.

The instructions for transferring the funds were allegedly issued by a bank employee, Gokulnath Shetty, using an international messaging system for banking, called SWIFT platform, and without making their subsequent entries in the PNB's internal banking software, thus bypassing scrutiny in the bank, they said.