The Treasury Department said on Wednesday that it would repurchase up to $6 billion of its own long-dated debt, fulfilling a promise to at least double its debt buybacks as it seeks to rein in rising bond yields. The Treasury’s purchases are designed to reduce the availability of longer-dated bonds, pushing their prices higher and lowering yields. Treasury yields underpin borrowing costs throughout the economy, with higher interest rates crimping the affordability of housing, cars and other items bought on credit for many Americans.

The purchases, scheduled to be carried out Thursday afternoon, will increase the size of the Treasury’s buyback operation for debt maturing in 10 to 20 years to $6 billion, from $2 billion last month. Bond yields, however, rose shortly after the announcement, suggesting investors were underwhelmed by the size of the purchases. The 10-year Treasury yield rose 0.05 percentage points to roughly 4.85 percent, its highest level since the end of 2023.

The 20-year yield rose by a similar amount, to 5.3 percent, also its highest since late 2023. Speaking at Southern Methodist University on Tuesday, Scott Bessent, the Treasury secretary, defended plans to make the bond market move. He argued that markets were misreading the fundamental dynamics of the U.S. economy and noted that, for investors, American bonds had outperformed the bond markets of many other countries.

A former hedge fund manager who once tried to capitalize on fast-moving market anomalies, Mr. Bessent said his job as Treasury secretary is to ensure that markets are not misreading the fundamental dynamics of the economy.