Treasury Secretary Scott Bessent promised to go big to bring down bond yields. So far, his efforts still aren't big enough.

U.S. government bond yields climbed to fresh multiyear highs on Wednesday after the Treasury Department said it would repurchase up to $6 billion of longer-term debt at a buyback operation on Thursday, disappointing some investors who had thought that a larger amount was possible.

The Treasury had already said last month that it would at least double buybacks of longer-term bonds from a maximum of $2 billion per operation, expanding a program that was originally intended to improve liquidity in older, less frequently traded securities. Investors widely viewed that as an effort to contain borrowing costs since it came at a time when longer-term yields were approaching key thresholds.

Calling himself the "nation's top bond salesman," Bessent stated early in Trump's first term that lowering bond yields was a priority for the administration-a goal that has been repeatedly thwarted by factors such as the Iran conflict and tariffs, as well as by events outside the administration's control.

Overall, the former hedge-fund manager has taken an unusually aggressive approach to managing markets, including interventions to support the Argentine peso and Japanese yen.

In recent days, he has only dialed up his rhetoric. "I have asymmetric information. I am the house now," he said on Tuesday, in reference to the recent intervention in the yen, adding: "You can bet against me if you want."

But Wednesday's bond-market move, while relatively contained, demonstrated the risk of that kind of talk-namely, that the Treasury secretary might be setting targets that could be difficult to meet.

"Any move to bring yields down with buybacks is falling flat," said Jack Ablin, chief investment strategist at Cresset Capital. "It's papering over a problem."

Some investors said the Treasury Department is now in a difficult position because expectations for the size of repurchases are high, yet it could be difficult to truly impress the market without relaxing guidelines that it will only buy bonds at prevailing market prices.

That has "put the Treasury into a lose-lose situation with these buybacks," said Leah Traub, a fixed-income portfolio manager at Lord Abbett.

Treasury yields, which rise when bond prices fall, have climbed for much of the summer, pushing yield on 30-year bonds to their highest levels in 19 years. One big reason: the war-powered surge in fuel costs. On Wednesday, Brent crude prices climbed above $100 for the first time since July as tensions escalated in the Middle East.

That is stoking fears of stubbornly high inflation that pushes the Federal Reserve to raise interest rates as soon as its meeting next week. But traders add that there are a number of other factors pushing yields higher, such a solid labor market, swollen U.S. budget deficits and a deluge of tech-company debt issuance competing for investors' cash.

The bond rout could put pressure on stock prices, which have slumped in the first few days of what has historically been a rough month for equities. All three major U.S. stock indexes fell Wednesday, with the Dow Jones Industrial Average dropping 0.8%, the S&P 500 falling 0.5%, and the Nasdaq composite losing 0.6%.

High bond yields lift the cost of borrowing for consumers and companies, raising mortgage rates and ultimately weighing on corporate profits. Climbing rates could also convince more investors to park their money in ultrasafe Treasurys versus riskier stocks.

"As yields rise, it competes against equities for capital," Ablin said. "There may be a lot of investors switching gears."

Recent moves in the bond market haven't been all bad for Bessent. Yields generally have climbed. But yields on longer-term bonds have risen less than those on shorter-term notes, a sign that the market is being driven more by economic data and expectations for interest-rate increases than by more unpredictable factors such as concerns about the Fed's independence or the size of the deficit.

Yields on European government bonds, which are more sensitive to energy prices, have climbed more sharply than those on U.S. bonds in recent weeks, and did so again on Wednesday. Treasury yields retreated from their session highs after a $39 billion auction of 10-year notes met with healthy demand from investors.

The yield on the benchmark 10-year note settled at 4.836%, according to Tradeweb, up from 4.805% Tuesday though down from 4.853% earlier in the session-its highest intraday level since November 2023.

Michael Lorizio, head of U.S. rates and mortgage trading at Manulife Investment Management, said that bonds could get a boost on Thursday if the buyback operation goes smoothly, noting that a $6 billion total was still generally in line with what most investors had anticipated.

Still, he said, questions about the size of future buyback operations could also weigh on the market over the longer-term.

The market "really does not love uncertainty, and this just adds greater uncertainty to some degree," he said.