* Fed to keep buying $85 bln per month in assets
* Aetna shares rise after outlook
* Dow up 0.5 pct; S&P 500 up 0.7 pct; Nasdaq up 0.4 pct
NEW YORK, Dec 12 (Reuters) - U.S. stocks climbed to session highs on Wednesday after the U.S. Federal Reserve announced a new stimulus plan at the end of its two-day monetary policy meeting in its latest attempt to kickstart a struggling economy.
The central bank replaced a more modest stimulus program due to expire at year-end with a fresh round of Treasury purchases that will increase its balance sheet, as was widely expected. It committed to monthly purchases of $45 billion in Treasuries on top of the $40 billion per month in mortgage-backed bonds it started buying in September.
"The market was expecting this - they basically stopped doing the Operation Twist, and it looks like they are going to be doing $45 billion a month in Treasury purchases, which was the expectation," said Troy Logan, managing director and senior economist at Warren Financial Service, in Exton, Pennsylvania.
"This was expected, and the market is waiting for the year-end 'fiscal cliff' issue to be solved, so what we have to do is have confidence our political system can actually make a functional decision."
U.S. House of Representatives Speaker John Boehner said on Wednesday "serious differences" remain with President Barack Obama in talks to avert the "fiscal cliff" of steep tax hikes and budget cuts set for the end of the year.
The S&P 500 was up for a sixth straight day, its longest winning streak since August, although gains have been less than 0.5 percent per day, on average, in part due to uncertainty over the cliff negotiations.
The Dow Jones industrial average gained 71.44 points, or 0.54 percent, to 13,319.88. The Standard & Poor's 500 Index rose 10.19 points, or 0.71 percent, to 1,438.03. The Nasdaq Composite Index advanced 12.29 points, or 0.41 percent, to 3,034.59.
Shares of Aetna, the third-largest U.S. health insurer, jumped 4.8 percent to $46.61 a day after the company gave a higher forecast for profit and revenue growth next year.
Wal-Mart Stores Inc fell 2.3 percent to $69.24 as the biggest drag on the Dow after India's government announced an inquiry into the company's lobbying practices.