The largest banks in the United States bolstered their equity capital levels at the end of 2015 according to a stricter measure by the Federal Deposit Insurance Corp., though a leading U.S. bank regulator lamented a move toward relaxation of international capital standards in recent days.

The eight globally systemically important, or "too big to fail" banks in the U.S. saw their average equity capital level rise from 5.73 percent to 5.97 percent between June and December, according to the FDIC's Global Capital Index, a measure that takes care to include exposure to derivatives as part of the assets of a bank.

"It is important that this progress continue to a point where they reach solid levels of tangible capital to effectively move the cost of downside risk from the public to the firms," said Thomas Hoenig, vice chairman of the FDIC.

Hoenig noted that even as their equity capital levels have increased relative to their foreign banking counterparts, lending and the overall economy have grown faster in America than in Europe. Bankers argue that higher capital standards could make them less competitive compared to foreign banks with lower standards.

The outspoken regulator and former president of the Federal Reserve Bank of Kansas City also criticized the Basel Committee on Banking Supervision, which last week proposed a new method for banks to calculate their exposure to derivatives that could wind up lowering the amount of capital lenders need.

"I find it unfortunate that the Basel Committee last week has recommended changes to the international leverage ratio that would significantly weaken it — even if its capital requirements are increased — by turning it into a risk-weighted measure," Hoenig said. "Doing so would significantly dilute the effectiveness of the most reliable measure of bank capital and result in increased leverage that does not serve the financial system, broader economy or even the firms well over the full course of an economic cycle."

The FDIC's Global Capital Index showed that Morgan Stanley, the Bank of New York Mellon and Goldman Sachs have the lowest leverage ratios of the American banks that are systemically important, with none rising above 5 percent.

Wells Fargo's ratio was 8.2 percent, well above the other seven institutions.

U.S. Bank, which is considered systemically important for the domestic but not the global economy, had a ratio of 8.15 percent, according to the FDIC.

Hoenig has called for all banks to maintain a leverage ratio of 10 percent.

Higher capital requirements are one of the ideas on the agenda at the Federal Reserve Bank of Minneapolis, which under its new president, Neel Kashkari, is studying the "too big to fail" phenomenon. Kashkari promises by year's end to deliver a recommendation to Congress and the American public.

Adam Belz • 612-673-4405 Twitter: @adambelz