NEW YORK (AFP) - US stocks dropped a fifth straight day on Wednesday as the Federal Reserve kept interest rates unchanged amid concern about Britain's upcoming vote on whether to exit the European Union.

The June 23 Brexit vote "could have consequences for economic and financial conditions in global financial markets," said Fed chairman Janet Yellen.

"If it does so, it could have consequences in turn for the US economic outlook."

Wall Street had been in modestly positive territory all day, but fell into the red in the final half hour after Yellen's news conference.

Some investors likely sold after the Fed announcement failed to spur a major rally, said Mace Blicksilver, director of Marblehead Asset Management.

The Dow Jones Industrial Average shed 0.2 per cent to 17,640.17.

The broad-based S&P 500 lost 0.2 per cent to 2,071.50, while the tech-rich Nasdaq Composite Index fell 0.2 per cent to 4,834.93.

Shares of metals producers rose after reports that China plans to increase stockpiles of base metals boosted commodity prices. Copper and gold producer Freeport-McMoRan climbed 7.7 per cent, while aluminum producer Alcoa gained 3.9 per cent.

Steel company Nucor gained 1.9 per cent after forecasting second-quarter earnings above analyst expectations. United States Steel surged 8 per cent.

Whole Foods Market tumbled 5 per cent after the US Food and Drug Administration reported "serious violations" in its food-handling practices at a facility in Massachusetts.

Among technology shares, Cisco Systems lost 1.1 per cent, Intel 1.7 per cent and Apple 0.3 per cent.

But Twitter rose sharply for a third straight day, adding another 3.9 per cent on speculation it could be acquired.