Moody's Investors Service (MCO) lowered its outlook for the long-term ratings of some of the world's largest financial institutions.

Analysts placed Bank of America (BAC), Citigroup (C), Goldman Sachs (GS), JPMorgan Chase (JPM), Morgan Stanley (MS), and Royal Bank of Canada (RY) on a review for a possible credit ratings downgrade.

The ratings agency also extended its review of 11 other firms, including seven large European banks.

"These firms face challenges that are not fully captured in their current ratings," Moody's analysts said. "Capital markets firms are confronting evolving challenges, such as more fragile funding conditions, wider credit spreads, increased regulatory burdens and more difficult operating conditions."

Moody's said the banks have tempered the effects of the challenging market environment through changes to business models and higher capital and liquidity ratios, but the pressure mounts "to generate the level of return on equities expected by shareholders."

"The rapidly changing risk positions expose these firms to unexpected losses that can overwhelm the resources of even the largest, most diversified groups," according to analysts.

The average credit rating for the banks is A2 on Moody's scale, and analysts said the average would likely move to the Baa range, if the reviews result in downgrades.