Oil and banking stocks were particularly hard-hit, with Chevron off 7%, Bank of America 6.4% and Citigroup 5.8%.
WASHINGTON—Stock losses on Wall Street mounted to more than 3% at midday Wednesday driven in part by another plunge in crude oil prices.
At about 12:25 pm (1725 GMT) the S&P 500 was down 3.5% at 1,814.67, and the Dow Jones Industrial Average was off 3.3 percent at 15,483.53.
The Nasdaq Composite meanwhile lost 3.5% at 4,318.87.
The losses accelerated after crude prices sank to a fresh 12-year low, with the New York benchmark WTI plunging nearly seven percent to $26.50 a barrel.
Amid a broad selloff of large-cap shares, oil and banking stocks were particularly hard-hit, with Chevron off 7%, Bank of America 6.4% and Citigroup 5.8%.
In late afternoon deals, U.S. benchmark West Texas Intermediate (WTI) for February delivery tanked to $26.30 per barrel, a level last seen in early May 2003.
The contract later stood at $26.50, down $1.96 from Tuesday's closing level.
The global oil market has collapsed further since the International Energy Agency warned Tuesday that the oil market could "drown in oversupply."
Prices have crashed about 75% since mid-2014, hit by a perfect storm of a stubborn supply glut, the slowing global economy and the rebounding US dollar.
"A stronger dollar continues to present significant headwinds and supply increases show little sign of letting up any time soon," said Sucden analyst Kash Kamal.
Copyright Agence France-Presse, 2016