Debt market sources said losses could range from $500m to $1.5bn. However, CSFB dismissed the speculation as "preposterous" and "way off beam".
A senior CSFB executive in New York said last night: "Anyone who is speculating about our positions doesn't know our positions. Only we know our positions. Until we know what form Monday's restructuring takes, it is premature to say what, if any, losses CSFB has sustained."
The executive said that in the event of Monday's restructuring having a material affect on the bank's financial position, it would make a statement.
The fears, which have sparked concern among staff in London, Zurich and New York that bonuses may be cut because of the dramatic impact on investment banking profits this year, came amid warnings from Moscow that some of the country's 20 leading banks could now fail.
Shares in Credit Suisse, its Swiss banking parent, have taken a battering in Zurich over the last few days. They fell nearly 4 per cent yesterday.
Knowledge of the losses is widespread in emerging market trading circles: traders said they believed them to be "substantial". "CSFB is haemorrhaging," said one Russian debt trader yesterday. "They have some horrible secondary positions." Another said: "They are the market leader: you would expect them to have suffered losses."
The bank, which was the first major Western investment bank to set up in Moscow after the fall of communism, is by far the biggest player in the $40bn Russian debt market. The firm employs 320 people in Moscow with satellite offices in Kiev and Tashkent.
Other Western investment banks have also been badly burnt over the past few days by the payment halt, although none have been hit to the same extent. The other major players in the short-term government debt market are ING Barings and JP Morgan.
Analysts say CSFB accounts for more than half of the foreign holdings of GKOs, the short-term rouble-denominated government bonds. Altogether, foreign banks are thought to account for a quarter of the total market.
The size of CSFB's losses would explain why the bank has been leading efforts to persuade the Russians to honour their obligations to foreign investors. The Russians withdrew their original proposals for restructuring their short-term debt obligations on Wednesday after being warned that they were highly discriminatory towards foreigners and would "permanently damage the Russian economy".
CSFB kept up its pressure on the Russian authorities yesterday with a further warning about the damage the planned debt restructuring would do to the banking system by keeping afloat banks that deserved to go to the wall.
Rival SBC Warburg was yesterday appointed to handle the debt restructuring.
The crisis yesterday spread to the Ukraine. Dealings on the Kiev foreign exchange were halted at the request of the central bank in a move expected to be followed by a devaluation in the country's currency, the hryvnia.