NEW YORK (REUTERS) - United States (US) stocks fell on Monday, pulling back before this week's Federal Reserve meeting that could signal when the Fed is going to begin reducing its bond purchases aimed at helping the economic recovery.

Losses were led by the energy and financial sectors, with both the S&P energy index and S&P financial index down 0.8 per cent.

Shares of Southwestern Energy slid 3 per cent to $38.14 and shares of Noble Energy fell 2.1 per cent to $62.07 following as a decline in natural gas prices.

Several merger announcements helped to limit losses, along with news of a US$1 billion (S$1.26 billion) stock repurchase program by Caterpillar that pushed its stock up 1.1 per cent to $83.03, making it the biggest support for the Dow.

But the Fed's statement, which is due on Wednesday after a two-day meeting of the Fed's Open Market Committee, kept investors wary of buying. The statement will be scrutinised for hints on when the central bank may begin to scale back its massive bond-buying aimed at stimulating the economy.

Data this week includes July's payrolls report, another key event for the market.

"A lot depends on how (Fed policymakers) interpret the data and how they comment ahead of the employment report. We're looking for clues as to whether tapering is going to begin in September or not," said Mr Bucky Hellwig, senior vice president at BB&T Wealth Management in Birmingham, Alabama.

The Dow Jones industrial average was down 36.86 points, or 0.24 per cent, at 15,521.97. The Standard & Poor's 500 Index was down 6.32 points, or 0.37 per cent, at 1,685.33.

The Nasdaq Composite Index was down 14.02 points, or 0.39 per cent, at 3,599.14.

September is the most likely time for the Fed to begin paring its US$85 billion in monthly bond purchases, according to a July 22 Reuters poll of economists.

Some investors have worried that big gains in jobs numbers could be enough of an economic pickup to prompt an early end to the Fed's bond buying, a program which has helped stocks rally for much of this year.

But analysts have noted that signs of a stronger economy are more important for the market in the long run. The S&P 500 is up 18.2 per cent for the year so far.

Monday's data was less than upbeat. Contracts to purchase previously owned US homes fell in June, retreating from a more than six-year high touched in May as rising mortgage rates were starting to dampen home sales.

Merger activity could give equities support as big deals show that large investors see value in the market.

"It's interesting to me that you've got deal activity this time of the year because normally this is the time of the year when the markets are quite quiet," said Mr Dan Veru, chief investment officer of Palisade Capital Management in Fort Lee, New Jersey, which manages about US$4.5 billion in assets.

"To have mergers going on now probably bodes well that the fall is going to be a very active period." US drugmaker Perrigo agreed to buy Irish drug company Elan for US$8.6 billion.

US-traded Elan shares rose 3.5 per cent to $15.46. Perrigo was the S&P 500's worst percentage decliner, shedding 6.7 per cent to $125.17.

Hudson's Bay Co, operator of department store chains Lord & Taylor in the United States and The Bay in Canada, said it would buy luxury retailer Saks Inc for $16 per share. Saks shares rose 4.2 per cent to $15.95.

Shares in advertising groups jumped after Publicis and Omnicom said they would merge. Investors bet the deal would create an opening for rivals to poach defecting clients and potentially trigger more deals.

Omnicom shares were down 0.6 per cent to $64.75 while smaller rival Interpublic Group gained 4.7 per cent to $16.61.

Among the day's big gainers, shares of CF Industries Holdings Inc, the world's second-largest maker of nitrogen fertiliser, jumped 11.8 per cent to $202.30 after activist hedge fund Third Point LLC said it had acquired a stake.

In earnings news, Loews Corp, the hotel, energy and financial services conglomerate, posted a jump in second-quarter profit as revenue from its insurance arm, CNA Financial, increased nearly 13 percent. Shares of Loews ended up 0.02 per cent at $46.06.

Halfway through earnings season, 67.2 per cent of S&P 500 companies have beaten analysts' expectations - in line with the 67 per cent average beat over the last four quarters.

About 56 per cent of the companies have beaten revenue expectations, more than the 48 per cent of revenue beats in the past four earnings seasons but below the historical average.