International ratings agency Standard & Poor's (S&P) placed its ‘BBB-' long-term and ‘A-3’ short-term corporate credit rankings on Russian oil company TNK-BP on CreditWatch with positive outlook, the agency said in a statement.

“The rating actions follow an announcement by Russian state-controlled Oil Company Rosneft OJSC that it had agreed in principle to purchase a 50% stake in TNK-BP from BP PLC for $17.1 billion in cash and 12.84% of treasury shares. Rosneft also intends to buy another 50% of TNK-BP from the Russian consortium AAR for $28 billion,” the statement said.

Investors were cheered by the deals struck by Russian state-controlled oil giant Rosneft with British oil giant BP and the AAR consortium of Russian billionaire shareholders, to repurchase their 50-50 stakes in TNK-BP, Russia’s third-largest oil producer.

Rosneft and BP announced their deal on Monday, following a BP board meeting on Friday. Rosneft said last week it had made a provisional agreement to buy AAR's stake.

Meanwhile, Moody's Investors Service has placed the ratings of oil giant Rosneft and TNK-BP on review for downgrade following Rosneft's announcement it intends to acquire 100 percent of the Russian-British joint venture, Moody's said on Wednesday.