The U.S. Federal Reserve Board is proposing a tough new rule that pushes large banks to strengthen their liquidity.

The Fed unveiled a proposal Thursday that would, for the first time, create a standardized minimum liquidity requirement for large, internationally active banks and systemically important, non-bank financial firms that are designated by the Financial Stability Oversight Council (FSOC). Under the proposed rule, these firms would be required to hold minimum amounts of high-quality, liquid assets — such as central bank reserves, and government and corporate debt, that can be quickly and easily converted into cash — that’s at least equal to projected cash outflows minus projected cash inflows during a short-term stress period.

“Liquidity is essential to a bank’s viability and central to the smooth functioning of the financial system,” said Fed chairman, Ben Bernanke. “The proposed rule would, for the first time in the United States, put in place a quantitative liquidity requirement that would foster a more resilient and safer financial system in conjunction with other reforms.”

The proposed liquidity coverage ratio (LCR) would apply to large banks — generally defined as those with $250 billion or more in total consolidated assets, or $10 billion or more in on-balance sheet foreign exposure — and to systemically important, non-banks. The proposal also contemplates a less stringent LCR for smaller banks that are not internationally active, but have more than $50 billion in total assets.

The Fed notes that its proposal is based on a standard agreed to by the Basel Committee on Banking Supervision, but that it is more stringent in several areas, including the range of assets that will qualify, and the assumed rate of outflows of certain kinds of funding.

The proposed transition period is also shorter than contemplated by the Basel agreement, which the Fed says “reflects a desire to maintain the improved liquidity positions that U.S. institutions have established since the financial crisis.” Under the proposal, U.S. firms would begin the LCR transition period on January 1, 2015, and would be required to be fully compliant by January 1, 2017.

“Since financial crises usually begin with a liquidity squeeze that further weakens the capital position of vulnerable firms, it is essential that we adopt liquidity regulations to complement the stronger capital requirements, stress testing, and other enhancements to the regulatory system we have been putting in place over the past several years,” said Fed governor, Daniel Tarullo. “This rule would help ensure that the liquidity positions of our banking firms do not weaken as memories of the crisis fade.”

The proposal is out for comment until Jan. 31, 2014.