The agency, in a statement to Russia's state-run news agency RIA Novosti, said it gave a BBB- long-term rating to TNK-BP and an A-3 short-term for the joint venture.
"The rating actions follow an announcement by Russian state-controlled oil company Rosneft OJSC that it had agreed in principle to purchase a 50 percent stake in TNK-BP from BP PLC for $17.1 billion in cash and 12.84 percent of treasury shares," the statement read. "Rosneft also intends to buy another 50 percent of TNK-BP from the Russian consortium AAR for $28 billion."
AAR -- Afla-Acess-Renova -- represented the Russian half of TNK-BP. Both sides of the venture were at odds over corporate direction.
Moody's Investors Service said Wednesday it put the venture on a downgrade review following the announcements.