WASHINGTON (AP) — Treasury Secretary Timothy Geithner is putting pressure on the Securities and Exchange Commission to overhaul its rules for money-market mutual funds.

Geithner sent a letter Thursday to members of the Financial Stability Oversight Council seeking their help in pressuring the SEC to change its rules. Geithner, who heads the panel, said the changes are necessary to protect the system.

SEC Chairman Mary Schapiro has pushed for the changes. But she abandoned them last month because three of the agency's five commissioners oppose new rules.

The changes were sought after a big fund collapsed during the 2008 financial crisis. One proposed change would require funds have capital cushions, like banks.

The SEC would be required to adopt the council's recommended changes or explain why it failed to act.

Money-market funds are a linchpin of the economy, holding $2.7 trillion in assets. Individual investors, corporate treasurers and professional money managers rely on money funds to limit losses when stocks plunge.

The mutual fund industry has vigorously opposed an overhaul of money funds. The industry supported less comprehensive restrictions on the funds that the SEC adopted 2 ½ years ago. But industry leaders warn that further changes would make money funds so unattractive that investors could pull out of them altogether

Geithner said in his letter that if the SEC fails to act "in a timely and effective manner," the council should take its own steps. Those could include designating some money funds as firms that pose a threat to the financial system if they were to fail, he said. That would put those firms under close oversight by the Federal Reserve.

Without an overhaul of money funds, "our financial system will remain vulnerable to runs and instability," Geithner wrote.

A report issued by the oversight council in July identified money funds as one of the risks to U.S. financial stability. The collapse of the Reserve Primary Fund in 2008 created a scare that led the government to temporarily guarantee money fund assets so investors could be assured they would be protected from losses.

Members of the council also include Federal Reserve Chairman Ben Bernanke, Schapiro and Martin Gruenberg, acting chairman of the Federal Deposit Insurance Corp.