"It is not clear how the Fed wants to deal with bond yields," said Hirokazu Kabeya, chief global strategist at Daiwa Securities.
"The pace of rises in yields has been far faster than most people have expected and there's speculation the authorities may be starting to think about tightening their policy."
Euro Stoxx 50 futures fell 0.9% while Britain's FTSE futures edged down 0.5% lower.
The MSCI's ex-Japan Asian-Pacific shares lost 1.8% in early trade while Japan's Nikkei fell 2.2%.
E-mini S&P futures slipped 0.4% while the futures for the Nasdaq, the unequivocal leader of the post-pandemic rally, fell 0.7%, hitting a two-month low.
Tech shares are vulnerable because their lofty valuation has been supported by expectations of a prolonged period of low interest rates.
But the market is laser-focused on Powell, who is due to speak at a Wall Street Journal conference at 12:05 p.m. EST (1705 GMT), in what will be his last outing before the Fed's policy-making committee convenes March 16-17.
Many Fed officials have downplayed the rise in Treasury yields in recent days, although Fed Governor Lael Brainard on Tuesday acknowledged concerns over the possibility a rapid rise in yields could dampen economic activity.
In addition, anxiety is building over a pending regulatory change in a rule called the supplementary leverage ratio, or SLR, which could make it more costly for banks to hold bonds.
"The market is likely to be unstable until this regulation issue will be sorted out," said Masahiko Loo, portfolio manager at AllianceBernstein. "There aren't people who want to catch a falling knife when market volatility is so high."
In addition, the market will also have to grapple with a huge increase in debt sales after rounds of stimulus to deal with a recession triggered by the pandemic.
The issue is not limited to the United States, with the 10-year UK Gilts yield jumping back to 0.779%, near its 11-month high of 0.836% hit last week, after the government unveiled much higher borrowing.
Currency investors continued to snap up dollars as they bet on the U.S. economy outshining peers in the developed world in coming months.
The dollar rose to a seven-month high of 107.16 yen.
"U.S. dollar/yen has been on a one-way trajectory since the start of 2021," said Joseph Capurso, head of international economics at the Commonwealth Bank of Australia.
"The brightening outlook for the world economy is a positive for both U.S. dollar/yen and Australian dollar/yen."
Other safe-haven currencies were soft, with the Swiss franc flirting with a four-month low against the dollar CHF= and a 20-month trough versus the euro.
Gold hit a nine-month low of $1,702.8 per ounce on Wednesday and last stood at $1,719.
Other major currencies were little moved, with the euro flat at $1.2054.
Investor focus on a U.S. economic rebound was unshaken by data released overnight that showed the U.S. labour market struggling in February, when private payrolls rose less than expected.
Oil prices rose for a second straight session early on Thursday, as the possibility that OPEC+ producers might decide against increasing output at a key meeting later in the day underpinned alongside a drop in U.S. fuel inventories.
U.S. crude rose 0.6% to $61.64 per barrel.
(Additional reporting by Koh Gui Qing in New York; Editing by Sam Holmes, Richard Pullin & Simon Cameron-Moore) ((firstname.lastname@example.org; +1 646 223 6033;))