The U.S. dollar weakened sharply on Wednesday after the U.S. Treasury unexpectedly increased its buyback of long-term Treasury bonds. The dollar index, which measures the currency against a basket of major peers, fell as much as 0.85% intraday, its largest decline in three weeks, reaching its lowest level since mid-May. The dollar declined against every major currency, while the Swiss franc and Swedish krona posted the strongest gains and the yen also strengthened significantly. Market participants said the Treasury buyback, in which the government repurchases outstanding debt, helped relieve sustained selling pressure at the long end of the U.S. bond market and signaled that the department is closely monitoring volatility in the Treasury market.