Fidelity Investments, the nation's largest money market mutual fund manager, has sold all of its short-term U.S. government debt — the latest sign that investors are increasingly nervous about the possibility of a government default.

Money market portfolio managers at Fidelity Investments started selling off short-term U.S. government debt a couple of weeks ago, Nancy Prior, president of Fidelity's Money Market Group, said Wednesday. While Fidelity expects the debt ceiling issue to be resolved, the Boston-based asset manager said it has taken steps to protect investors.

"We expect Congress will take the steps necessary to avoid default, but in our position as money market managers we have to take precautionary measures," Prior said.

Fidelity, which manages $430 billion in money market mutual funds, has taken similar actions in the past. The most recent instance was in the summer of 2011, when the U.S. government came close to a default and Standard & Poor's downgraded the nation's credit rating, Prior said.

Prior said that Fidelity no longer holds any U.S. debt that comes due in late October or early November, the window considered by many investors to be the most exposed if the government runs out of money to pay its debts.

Money market funds are a significant part of the U.S. financial system. Individuals and institutional investors have roughly $2.685 trillion invested in the funds, according to data from the Investment Company Institute.

Money market funds are typically ultra-safe places to park money. They invest primarily in short-term debt that can be easily bought and sold, such as U.S. Treasurys or commercial paper, debt issued by large companies to fund their day-to-day expenses. In a money market fund, investors expect to get back every dollar they invest.

The U.S. Treasury has warned it will run out of money if Congress does not agree to raise a $16.7 trillion cap on borrowing by Oct. 17 and allow it to issue more debt.

The worry has other parts of the market showing signs of stress. Like Fidelity, other investors have tried to limit their exposure to U.S. government debt that comes due this month, with the heaviest selling occurring in one-month Treasury bills. The yield on the one-month T-bill jumped to 0.27 percent Wednesday, its highest level since the 2008 financial crisis. The yield was nearly zero at the beginning of the month.

Money market mutual fund managers don't want to be caught holding U.S. government debt that comes due around the time the government hits the debt ceiling. They fear that the government may not be able to pay back bond holders, said Gabriel Mann at the Royal Bank of Scotland Group.

"Investors are buying protection," Mann said, referring to growing demand for insurance against the U.S. defaulting on its debt __ a security known on Wall Street as a credit default swap.

Overnight interest rates in the repo market, used by banks to fund day-to-day lending, shot up to 0.12 percent Wednesday from 0.04 percent at the beginning of the month.

The increase is partly because some banks have stopped accepting some U.S. Treasurys as collateral, or are requiring more collateral, to borrow.

Not all investors are worried though.

"We're doing just the opposite ... probably buying what Fidelity is selling," Bill Gross, co-founder of PIMCO, the world's largest bond fund manager, said Wednesday in an interview with CNBC.

Gross said the odds are a million to one that the U.S. will not default on its debt.