WASHINGTON (AP) — The Federal Reserve on Thursday proposed that big banks keep enough cash, government bonds and other high-quality assets on hand to survive during a severe downturn on par with the 2008 financial crisis.

The proposal subjects U.S. banks for the first time to so-called "liquidity" requirements. Liquidity is the ability to access cash quickly.

The largest banks — those with more than $250 billion in assets — would be required to hold enough cash and securities to fund their operations for 30 days during a time of market stress. Smaller banks — those with more than $50 billion and less than $250 billion — would have to keep enough to cover 21 days.

Fed officials said the rules are stronger than new international standards for banks. Combined, the largest banks would have to hold an estimated $2 trillion in high-quality assets to meet the requirement. The banks already hold roughly 90 percent of that amount, according to the Fed.

The public has 90 days to comment on the proposal, which was approved unanimously by the Fed's 6-member board of governors. After that the rules would be phased in beginning January 2015.

"Liquidity is essential to a bank's viability and central to the smooth functioning of the financial system," Fed Chairman Ben Bernanke said. He said the new regime "would foster a more resilient and safer financial system in conjunction with other reforms."

The requirements were mandated by Congress after the financial crisis. They are part of new regulations that are intended to prevent another collapse severe enough to require taxpayer-funded bailouts and threaten the broader financial system.

Hundreds of U.S. banks received federal bailouts during the crisis. Among them were the largest financial firms, including JPMorgan Chase, Goldman Sachs, Citigroup, Bank of America and Wells Fargo. The banking industry has been recovering steadily since then, with overall profits rising and banks starting to lend more freely.

The health of the financial sector surfaced during this month's stalemate over increasing the government's borrowing authority. Worries about a possible U.S. debt default raised questions about how well prepared banks and companies were to deal with a crisis.

Several Fed governors said Thursday that related requirements also are being considered, such as a mandate for large banks to test their liquidity under stress conditions.

The requirements proposed Thursday are "an important step forward but there's still more work to do," said Fed Vice Chair Janet Yellen, who has been nominated by President Barack Obama to succeed Bernanke as chairman next year.