US five-year and seven-year yields dropped to three-week lows as well, while those on two-year notes fell to a nearly two-week trough.

"Declines in the equity market are reflective of a potentially deteriorating US growth outlook, or increasing uncertainty about the growth outlook which would correspond to a lower path of policy for Treasuries," said Jon Hill, interest rates strategist, at BMO Capital in New York.

The benchmark S&P 500 was on a six-day losing streak and has fallen in 14 of this month's 18 trading days on worries ranging from rising borrowing costs and bond yields to Italy's budget, and upcoming US midterm elections.

Stock volatility has raised some concerns that equity markets and the economy may not be as strong as previously thought, which could temper the Federal Reserve's eagerness to raise interest rates. Investors also fear growing inflation pressures.

But on Wednesday, Cleveland Federal Reserve President Loretta Mester, a voting member of the Federal Open Market Committee this year, said there were no signs of a pending recession.

Treasuries also garnered bids as police on Wednesday intercepted suspected pipe bombs sent to former US President Barack Obama, former presidential candidate Hillary Clinton and other high-profile Democrats, in what New York officials described as an act of terrorism.

Aside from US politics, growth concerns about China, disappointing corporate earnings and tensions between Italy and the European Union over Italy's spending plans sent stock markets tumbling on Tuesday, and prompted demand for low risk US government bonds.

In afternoon trading, benchmark 10-year note yields fell to 3.105 percent from 3.166 percent late on Tuesday. Ten-year yields earlier fell to 3.102 percent, a three-week low.

US 30-year yields dropped to 3.333 percent from Tuesday's 3.364 percent.

On the short end of the curve, US two-year yields declined to 2.835 percent, its lowest since Oct. 12. They were last at 2.838 percent, down from 2.892 on Tuesday.

Wednesday's lackluster US five-year auction lifted intermediate yields off their lows. The note picked up a high yield of 2.977 percent, slightly up from the expected yield at the bid deadline. Bids totaled $89.8 billion for a soft 2.30 bid-to-cover ratio, a gauge of demand, down from both the 2.39 last month and the 2.48 average.

Direct bidders also took 1.9 percent, the smallest since 2009, according to Action Economics.