"PMO needs to answer on this issue," party's chief spokesman Randeep Surjewala remarked.
He was asked to react to a report in a Calcutta daily, The Telegraph, which spoke of a British arms agent wanted by Indian investigators in the scam of making such an allegation.
Party General Secretary Digvijay Singh took to Twitter to pose a question to Modi on the matter.
Christian Michel, the 54-year-old agent, has made the allegations in a letter to the International Tribunal of the Law of the Seas in Hamburg and the Permanent Court of Arbitration (PCA) at The Hague where Italy and India are battling legally over murder charges against the marines.
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Michel has claimed Modi made the offer at a secret "brush-by" meeting with Italian Prime Minister Matteo Renzi during the UN General Assembly in September 2015 when both leaders were in New York.
The Telegraph, which ran the report, said it could not independently verify Michel's allegations.
"The charges are too ridiculous to comment on," Foreign Ministry spokesperson Vikas Swarup told the newspaper which shared Michel's letter with the government for its response.