NEW YORK, July 23 (Reuters) - The U.S. Treasury has ramped up sales of short-term bills this month as the government borrows more money, a strategy that has found plenty of willing buyers but has sparked debate about the risks of relying too heavily on near-term financing.

Growing federal deficits and higher interest payments have pushed U.S. borrowing needs sharply higher, leading Treasury to increase issuance of short-term debt that has been absorbed quickly by money market funds, the biggest buyer of bills.

Some analysts cautioned though that a prolonged reliance on short-term debt leaves the government vulnerable to shifts in interest rates because the debt must be refinanced more frequently than longer-dated bonds.

"If rates need to materially go up, the funding cost will be substantially higher because you have to refund significantly more when it's all in T-bills versus when it's further out the curve," said Dhiraj Narula, HSBC's U.S. rates strategist, referring to debts that have longer maturities.

A senior Treasury official in a statement said that since more than 75% of marketable debt is at a fixed rate issued with a two-year maturity or longer, "changes in short-term interest rates do not affect the vast majority of the government's interest costs."

Wells Fargo's macro strategist Angelo Manolatos said net bill issuance in July so far of roughly $270 billion already exceeded his forecast for the entire month of $256 billion. For the first half of 2026, net new issue was $143 billion, according to Treasury data.

The surge in July Treasury bill issuance reflects the government's need to rebuild its cash balance and finance seasonal spending, including what analysts said were higher-than-expected tariff-related refunds.

RISING BILL SUPPLY

Goldman Sachs said in a note that 2026's total bill supply was expected to reach $827 billion versus roughly $360 billion in 2025.

The Treasury has relied heavily on bills since 2023 after Congress suspended the debt ceiling. It had to rebuild its cash, with Treasury turning to bills because their issuance could be scaled up quickly.

U.S. Treasury Secretary Scott Bessent continued that policy when he took office in 2025, leaving coupon auction sizes unchanged to help contain borrowing costs. By relying more on bills, which generally carry lower yields than longer-dated securities, the Treasury can borrow at lower rates and limit interest expenses.

Bills now account for 22% of outstanding marketable debt, with notes and bonds at 78%. The Treasury Borrowing Advisory Committee would like to keep bill issuance at 15%-20%.

SHORTER AVERAGE US DEBT MATURITY

The average maturity of U.S. government debt - about six years - is shorter than Britain's and Japan's but broadly in line with most major developed economies. This matters because it determines how quickly higher interest rates feed through to government borrowing costs and how often the Treasury must refinance its debt, making it a key gauge of both fiscal risk and interest rate sensitivity.

Some analysts also say demand from money market funds, with assets at nearly $8 trillion, may not keep pace with the latest onslaught of new issuance at least for July due to seasonal flows.

Wells Fargo's Manolatos said fund inflows tend to be lower early in the quarter, noting that cash balances rose $152 billion on average during July and August in the last three years. But most of those inflows came in August.

"Money funds have also shed a lot of T-bills since the beginning of the year and inflows alone will not be enough to absorb bill supply," Manolatos noted, adding that T-bill holdings fell by $365 billion in the first half of 2026.

"They may have to move money out of other assets to buy these bills."

Some strategists also warned that relying heavily on bills could constrain Treasury's options in any future crisis.

During the COVID-19 pandemic, Treasury used bills almost entirely for its funding needs because it had to raise trillions of dollars quickly. The share of bills as a percentage of debt consequently jumped above 25% in that period.

"If you're running T-bills at 30% of marketable debt in good times, you don't have that same capacity when a crisis hits," said Zach Griffiths, CreditSights' head of macro and investment-grade strategy.

Still, Treasury will continue to emphasize bill issuance as investor appetite remains strongest at the front end of the curve given "strong money fund inflows while demand further out the curve is more tenuous with worries about high deficits," said Gennadiy Goldberg, TD Securities' head of U.S. rates strategy.