U.S. stocks fell as Treasury yields continued their march higher despite the Treasury Department’s extraordinary debt buyback operation, raising concerns that higher borrowing costs will throw a wrench into the bull market.

The Dow Jones Industrial Average shed 514 points, or 1%, weighed down by a 9% drop in Walmart shares. The S&P 500 lost 0.5%, while the Nasdaq Composite dropped 0.9%.

Bond yields advanced on Thursday, climbing back from Wednesday’s slide following the Treasury Department saying it will at least double repurchases of 10-, 20- and 30-year debt in the next few months. Treasury Secretary Scott Bessent told CNBC Thursday that the debt buyback operation could actually be larger than the $4 billion that was announced.

The 30-year Treasury bond yield added more than 5 basis points at 5.251%, while the yield on the 10-year Treasury note gained more than 5 basis points to 4.706%. The 30-year yield spiked to its highest level in nearly 20 years earlier this week.

Adam Phillips, managing director of investments at EP Wealth Advisors, is skeptical that the buyback program will be enough to provide any relief, however.

“This is not the cure to what ails the bond market. There are structural forces here at play that are really beyond the Treasury and the administration’s control,” he said, adding that the relief seen in the wake of past interventions have generally been short-lived. “You’re going to need to come at it with a little bit more force if it’s going to have staying power.”

Also weighing on equities, oil prices rose again amid increasing tensions between Iran and the U.S. When discussing Iran, President Donald Trump said in a Truth Social post Wednesday that the U.S. would begin the “MOST CRUSHING ECONOMIC OPERATION EVER TAKEN AGAINST ANY COUNTRY!”

“This will be Economic Warfare and Isolation on an unprecedented scale,” he wrote.

On that front, Bessent said in a Thursday interview with CNBC that the U.S. is going to impose the “toughest sanctions in history” against Iran.

West Texas Intermediate crude futures rose 2% to trade above $88 per barrel, while global benchmark Brent crude futures moved up 2% to above $93 a barrel.

A pullback in Walmart shares hit the broader market as well. The retail giant was pacing for its worst day in more than four years after its U.S. comparable sales missed analyst expectations, as did its adjusted earnings forecast for both the third quarter and full year.

Wall Street is coming off a winning session, with the S&P 500 snapping a three-session losing streak, as yields on longer-dated U.S. Treasurys pulled back from multi-year highs after the government unveiled a plan to ease pressure from a recent bond market rout.

— CNBC’s Tobias Burns contributed reporting.