(Recasts first paragraph, adds comments from U.S. Federal Reserve Chair Jerome Powell, updates prices and details) New York, March 4 (Reuters) - Wall Street slumped on Thursday and global stock markets declined after U.S. Federal Reserve Chair Jerome Powell repeated his pledge to keep credit flowing until Americans are back to work, pushing back at investors who have doubted if he can hold that promise after the pandemic. Benchmarket U.S. Treasury yields rose toward last week’s highs as Powell spoke, and the dollar jumped. Oil prices spiked as OPEC and its allies agreed to extend most oil output cuts into April, after deciding that the demand recovery from the coronavirus pandemic was still fragile. With COVID-19 vaccines rolling out and the government fiscal taps open “there is good reason to think we will make more progress soon” toward the Fed’s goals of maximum employment and 2% sustained inflation, Powell told a Wall Street Journal forum. But “even if that happens it will take substantial time,” Powell added. The Dow Jones Industrial Average fell 350.11 points, or 1.12%, to 30,919.98, the S&P 500 lost 47.21 points, or 1.24%, to 3,772.51 and the Nasdaq Composite dropped 246.59 points, or 1.9%, to 12,751.16. “This market has already been weak and was looking for another excuse to sell, said Dennis Dick, head of markets structure and a proprietary trader at Bright Trading LLC in Las Vegas, citing fear in equities markets for the past nine months. “Now, Powell gives them that excuse as well.” The pan-European STOXX 600 index lost 0.37% and MSCI’s gauge of stocks across the globe shed 1.56%, its third day of losses. Emerging market stocks lost 2.54%. MSCI’s broadest index of Asia-Pacific shares outside Japan closed 2.57% lower, while Japan’s Nikkei lost 2.13% to its lowest since Feb. 5. Worries about loftier U.S. bond yields have also hit global shares. Powell said the increase was “notable” but he did not consider it a “disorderly” move, or one that pushed long-term rates so high the Fed might have to intervene to bring them down. Benchmark 10-year notes last fell 22/32 in price to yield 1.5449%, from 1.47% late on Wednesday. They earlier touched their highest levels since a one-year high of 1.614% set last week on bets on a strong economic recovery aided by government stimulus and progress in vaccination programs. The cost of borrowing U.S. Treasuries in the overnight repurchase agreement, or repo market, went negative on Thursday, analysts said, amid the bond market sell-off, which pointed to stress in money markets. The 10-year UK Gilts yield was last at 0.733%, after touching 0.796% on Wednesday, near last week’s 11-month high of 0.836%. Germany’s 10-year yield was down 2 basis points to -0.31% after rising 5 basis points on Wednesday. Many Fed officials have downplayed the rise in Treasury yields in recent days, although Fed Governor Lael Brainard on Tuesday acknowledged that concerns over the possibility a rapid rise in yields could dampen economic activity. The specter of high U.S. bond yields also undermined low-yielding, safe-haven assets, such as the yen, the Swiss franc and gold. Currency investors continued to snap up dollars as they bet on the U.S. economy outperforming its peers in the developed world in coming months. The dollar index rose 0.564%, with the euro down 0.78% to $1.1968. The Japanese yen weakened 0.82% versus the greenback. Dollar/yen rose to 107.90, roughly a seven month high, while Sterling was last trading at $1.389, down 0.45% on the day. Other safe-haven currencies were weakened, with the Swiss franc dropping to a five-month low against the dollar and a 20-month trough versus the euro. Rising Treasury yields pushed non-interest-bearing gold down 0.9%. Spot gold dropped 0.8% to $1,697.56 an ounce, but still near a nine-month low. Investor focus on a U.S. economic rebound was unshaken by data released overnight that showed the labor market struggling in February, when private payrolls rose less than expected. Oil prices rose for a second straight session as OPEC and its allies agreed to extend most oil output cuts into April, after deciding that the demand recovery from the coronavirus pandemic was still fragile. U.S. crude recently rose 4.57% to $64.08 per barrel and Brent was at $67.00, up 4.57% on the day.