NEW YORK, July 16 (Reuters) - The Dow industrials .DJI and Nasdaq .IXIC dipped into negative territory while the S&P 500 .SPX fell further after a survey showed factory activity in the U.S. Mid-Atlantic region contracted for a 10th consecutive month in July, adding to worries about the economy's strength.

The Dow Jones industrial average was down 9.97 points, or 0.12 percent, at 8,606.24. The Standard & Poor's 500 Index was down 3.80 points, or 0.41 percent, at 928.88. The Nasdaq Composite Index was down 1.49 points, or 0.08 percent, at 1,861.41.

Before the data, the Dow and Nasdaq were mostly flat while the S&P was slightly lower. (Editing by Padraic Cassidy)