(Reuters) -- Asian stocks struggled and commodities fell broadly Thursday after the Federal Reserve ramped up monetary stimulus by expanding Operation Twist, but disappointed some investors who had been hoping for more aggressive measures.

The U.S. central bank, as expected, extended its program of selling short-term securities and buying longer-dated ones, a move aimed at driving down borrowing costs, but did not signal a third round of quantitative easing.

MSCI's broadest index of Asia Pacific shares outside Japan <.MIAPJ0000PUS> edged down around 0.2 percent, mirroring similar declines on Wall Street <.SPX> <.DJI>. <.N>

Japan's Nikkei share average <.N225> bucked the trend, rising 0.7 percent as the yen weakened against the dollar, which should help Japanese exporters.

The positive impact of a weaker yen should outweigh the disappointment about the U.S.'s economic outlook and the lack of more powerful stimulus, said Masayuki Doshida, senior market analyst at Rakuten Securities. But there won't be a sustained rally on the back of this.

The Fed also slashed its forecast for U.S. economic growth, hitting commodities sensitive to expectations for industrial demand.

Copper fell 1.1 percent to around $7,465 a ton and oil also lost ground. U.S. crude dropped 1 percent to $80.64 a barrel and Brent crude was down 0.6 percent at $92.11, its lowest in 18 months.

A Reuters poll showed Wall Street's top bond firms still see a 50 percent chance of a third bout of quantitative easing or QE3, under which the Fed effectively creates money to fund large asset purchases, to stimulate the economy.

Clearly, the tilt is to do more. QE3 is one of those options, said Julia Coronado, chief economist North America at BNP Paribas in New York.

The decision to hold off on QE3 for now boosted the dollar against the euro and the yen and also hit gold, which had been rising as investors betting on QE3 had bought the precious metal as a hedge against currency depreciation.

The euro was down 0.3 percent around $1.2665 on Thursday, while gold fell 0.2 percent to just above $1,600 an ounce.

(Editing by Richard Pullin)