State-owned Rosneft on Monday unveiled a deal to buy TNK-BP, Russia's third-largest oil producer.
It is buying the 50 per cent stake of British oil company BP for $17.1 billion in cash and a 12.84 per cent share of Rosneft.
It is buying the other half for $28 billion from a group of Russian billionaires.
Moody's warned Rosneft and TNK-BP on Wednesday that the deal, estimated by Rosneft to be worth $61 billion, could hit both companies' credit ratings.
The agency said its further action would depend on the structure of the deal's financing as well as Rosneft's strategy regarding TNK-BP's corporate governance.