Traders work on the floor of the New York Stock Exchange, December 12, 2012.

US stocks mostly gained on Wednesday, but the moves were slim before the US Federal Reserve's expected announcement of a new stimulus plan at the end of its two-day monetary policy meeting.

The Fed is expected to detail a new round of bond buying later on Wednesday to boost a fragile economic recovery threatened by political wrangling over the government's budget. The Federal Open Market Committee's monetary policy decision is expected around 12:30 p.m. ET (19:30 SA time)

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 “fiscal cliff” negotiations.

The Fed looks certain to extend purchases of mortgage-backed debt and replace another expiring stimulus program, and the FOMC is likely to reiterate that it will keep buying bonds until the labor market improves substantially.

“The market is waiting on the Fed, anticipating the open check to be out again, but we will see. Anything less than enormous will be disappointing to the market,” said Joe Saluzzi, co-founder of New Jersey-based brokerage Themis Trading LLC.

Shares of Aetna, the third-largest US health insurer, jumped 4.3 percent to $46.39 a day after the company gave a higher forecast for profit and revenue growth next year. Late on Tuesday, Aetna forecast higher operating earnings in 2013 and revenue growth of 9 percent as the health insurer's medical plans add more members.

The Dow Jones industrial average edged up 7.44 points, or 0.06 percent, to 13,255.88. The Standard & Poor's 500 Index rose 2.95 points, or 0.21 percent, to 1,430.79. But the Nasdaq Composite Index slipped 2.98 points, or 0.10 percent, at 3,019.32.

A slide in Wal-Mart Stores shares accounted for the biggest point drag on the Dow at midday. Wal-Mart's stock fell 2.3 percent to $69.26 after India's government announced an inquiry into the company's lobbying practices.

With just two weeks of trading left this year, the S&P 500 is up about 14 percent so far. It ended the year flat in 2011.

Shares in Berkshire Hathaway shot up 2.4 percent to $133,975 after the company announced a $1.2 billion stock repurchase and raised the price at which it is willing to buy back shares. The shares were temporarily halted ahead of the announcement.

Asset manager BlackRock said it expected the S&P 500 to hit new heights in 2013 and reach 1,600 by the end of the year, surpassing its previous peak of 1,576.09 in 2007.

Negotiations intensified on Wednesday to avoid the automatic spending cuts and tax hikes of the “fiscal cliff” that would set in next year, as President Barack Obama and US House of Representatives Speaker John Boehner spoke by phone on Tuesday after exchanging new proposals.

But Boehner said there were still “serious differences” between them over how to avoid the cliff on Wednesday.

“I think that concerns are still lingering about the ability of Congress to get something done on the fiscal cliff, and that's not making people very confident,” said Michael James, senior trader at Wedbush Morgan in Los Angeles.

3M Co pointed to a profit rise of about 8 percent next year as the US economy continues its slow recovery. The stock was down 0.75 percent at $92.97.

Costco Wholesale Corp posted a better-than-expected 30 percent rise in quarterly profit, but its stock was nearly flat at $98.41, up just up 0.1 percent. - Reuters