The US Securities and Exchange Commission is investigating the sale of $850m in bonds issued by Mozambique by Credit Suisse, Russia's VTB Group and BNP Paribas.
This is according to a report in the Wall Street Journal.
The southern African nation said in October its debt was unsustainable and the International Monetary Fund suspended assistance to Mozambique when evidence of $2 billion in secret loans emerged.
The bonds were sold in 2013 to finance Mozambican state-owned fishing company EMATUM's plans to develop tuna fishing in the impoverished nation.
But the government later said it had also bought military equipment with the funds.
VTB Group's press office said in a statement today that it executed the transactions with Mozambique state-owned companies "in compliance with appropriate policies".
"The government of Mozambique confirmed to us that they were following the necessary internal and external legislation and that comprehensive information on the transactions was disclosed to creditors and investors," VTB said, adding it followed strict internal policies in these local deals.
Credit Suisse and BNP Paribas have declined to comment.
The SEC in November asked bondholders for documents provided by Credit Suisse, VTB and BNP Paribas during the sale of the bonds, the Wall Street Journal reported, citing a letter sent to bondholders.
The letter also asked investors to share with the SEC all communications with the banks related to the bonds.
Two emerging market investors told Reuters they were aware the SEC is investigating the EMATUM loan.
Financial watchdogs from Switzerland and Britain are also looking into Credit Suisse's and VTB's involvement.
Meanwhile, Mozambique's state news agency said yesterday that the country would pay government employees only half their annual bonus.