The loans are in addition to an $850 million 'tuna bond' issued in 2013 and restructured last month because the southeast African nation was struggling to meet repayments.
The International Monetary Fund (IMF) said last week it had got wind of more than $1 billion of undisclosed borrowing, although Finance Minister Adriano Maleiane dismissed the allegations and spoke of "some confusion".
Prime Minister Carlos Agostinho do Rosario then led a delegation to the United States to see IMF Managing Director Christine Lagarde to explain the borrowing and patch up tattered relations with the international lender.
"We're confident that we're not going to find anything else," the IMF source told Reuters, adding that Rosario's visit had gone some way to mending relations. "But we can't just go back to where we were. That takes time."
The extra borrowing pushes Mozambique's foreign debt to $9.64 billion, according to the new tuna bond prospectus - a level now "very close to unsustainability," the source added.
Proindicus, a state firm owned by the Ministries of Interior and Defence and the State Security and Intelligence Service, had been lent $504 million by Credit Suisse and $118 million by Russia's VTB, the source said.
According to a February 2013 Credit Suisse document obtained by Reuters, the money was earmarked for acquiring high-speed naval interceptors, radar stations, off-shore patrol vessels and aircraft. Credit Suisse has declined to comment on the document.
Another loan of $535 million went to Mozambique Asset Management, another state company set up to build a shipyard in the northern city of Pemba, the source said.
Pemba is near off-shore natural gas fields being explored by Anadarko (>> Anadarko Petroleum Corporation) and Eni (>> Eni SpA) that are thought to contain 180 trillion cubic feet of gas - enough to supply Germany, Britain, France and Italy for nearly two decades.
In addition, the Interior Ministry had borrowed $130-$200 million from an unidentified bilateral lender, the source added.
A foreign debt level of $9.64 billion is equivalent to 77 percent of GDP, a level that will become difficult to pay if its currency, the metical <MZN=>, extends last year's 40 percent decline against the dollar.
The failure of Maputo, Credit Suisse and VTB to disclose the extra borrowing during negotiations to reschedule the tuna bond has infuriated investors, some of whom have threatened to sue.
However, the IMF source said investors should observe patience since reigniting tensions would be more likely to trigger a default that could jeopardise Mozambique's ambitions to become one of the world's top three liquefied natural gas producers.
"There will be chaos," the source said. "The authorities will not be able to pay, the investors will get screwed, the debt rating will move into default and then the companies - Eni and Anadarko - will not be able to get project finance."
One bond-holder said some creditors were still unlikely to take it lying down, although the difficulties of taking legal action in a jurisdiction as murky as Mozambique could cause them to hang fire.
"I believe some creditors will potentially claim that the exchange was misleading," the bond-holder, who did not wish to be named, said.
"Then again, there is a cost there as well. I don’t really know any precedent where something like that has happened. This is a very unusual situation."
(Additional reporting by Karin Strohecker; Editing by Joe Brock and Richard Balmforth)
By Ed Cropley