WASHINGTON (AP) ― Federal regulators are requiring big banks to keep enough high-quality assets on hand to survive during a severe downturn, the latest move under congressional mandate to lessen the likelihood of another financial meltdown.

The Federal Reserve adopted rules on a 5-0 vote Wednesday that will subject big U.S. banks for the first time to so-called “liquidity” requirements. Liquidity is the ability to access cash quickly. The Federal Deposit Insurance Corp. and the Treasury Department’s Office of the Comptroller of the Currency adopted the rules later in the day.

Comptroller Thomas Curry, who also is a member of the FDIC board, said the new requirements “will help ensure that a banking organization’s cash resources, and not taxpayers’ money, will provide the support necessary for it to withstand short-term funding stress.”

At a meeting of the Fed governors, Chair Janet Yellen called the rules a “very important regulation that will serve to strengthen the resilience of internationally active banking firms.”

The 15 largest banks ― those with more than $250 billion in assets ― will have to hold enough cash, government bonds and other high-quality assets to fund operations for 30 days during a time of market stress. Smaller banks ― those with more than $50 billion but less than $250 billion in assets ― will have to keep enough to cover 21 days. Banks with less than $50 billion in assets and nonbank financial firms deemed by regulators as posing a potential threat to the system will not be subject to the requirements.

Separately, regulators also are proposing to give banks leeway in requiring collateral from companies that use derivatives to guard against price swings.

The liquidity rules for banks will begin to take effect in January, and the requirements will be phased in over two years.

Fed officials say the rules are stronger than new international standards for banks. Combined, the largest banks will have to hold an estimated $2.5 trillion in high-quality assets to meet the requirements. The banks already hold all but about $100 billion of that amount, according to the Fed. Banks have sharply improved their cash-ready holdings in the past few years, and about 70 percent now meet the full requirement, the Fed estimates.