U.S. stocks extended their recent selloff on Tuesday, with the S&P 500 hitting a three-week low, as energy shares dropped with oil prices and retailers including Target and Kohl's sank after weak earnings and forecasts.Target Corp shares slumped 9.5 percent after third-quarter profit missed analysts' estimates. The company's investments in its online business, higher wages and price cuts hurt margins.Department store operator Kohl's Corp shed 7.8 percent after its full-year profit forecast fell below expectations.Warnings from retailers added to caution for investors, already on edge over recent sharp losses in technology shares, a slowdown in global growth, peaking corporate earnings and rising interest rates."Despite what has been a pretty good earnings season, people are looking ahead to next year and are worried about a slowdown," said Mark Kepner, equity trader at Themis Trading in Chatham, New Jersey."It's sell first, ask questions later at this point." The S&P 500 and Dow slipped into negative territory for the year, and the Nasdaq fell to its lowest level in more than seven months.The S&P energy index tumbled about 3 percent and led sector losses as U.S. oil prices ended the day down 6.6 percent amid concerns about rising global supplies. The S&P 500 retail index lost 2.3 percent, on track for its eighth straight session of losses.The Dow Jones Industrial Average fell 513.95 points, or 2.05 percent, to 24,503.49, the S&P 500 lost 46.72 points, or 1.74 percent, to 2,644.01 and the Nasdaq Composite dropped 119.76 points, or 1.7 percent, to 6,908.72.Among other retailers, home improvement chain Lowe's Cos Inc fell 5.3 percent after it unveiled further plans of restructuring in the face of worse-than-expected sales numbers.TJX Cos Inc slipped 3.8 percent after the discount retailer's holiday-quarter earnings forecast came in largely below estimates. Smaller rival Ross Stores fell 8.1 percent after it forecast fourth-quarter same store sales below analysts' expectations.Apple Inc shares fell 4.5 percent as concerns lingered over slowing demand for iPhones. The stock, which has led the market through much of its long bull run, is at its lowest level since early May.Signs of cooling demand for iPhones have wide-ranging implications for technology and internet companies. Should Apple's loss hold through the day, its shares would have lost more than 20 percent of their value, or around $250 billion, since its October 3 record closing high. Goldman Sachs trimmed its price target on Apple for the second time in just over a week, saying the balance of price and features in the new iPhone XR may not have been well-received by users outside of the United States.Declining issues outnumbered advancing ones on the NYSE by a 5.85-to-1 ratio; on Nasdaq, a 3.10-to-1 ratio favored decliners.The S&P 500 posted 20 new 52-week highs and 41 new lows; the Nasdaq Composite recorded 10 new highs and 263 new lows.