U.S. stock index futures indicated a lower open Thursday, following rate decisions by the U.S. Federal Reserve and the Bank of Japan.
The Federal Reserve kept rates unchanged at its June meeting. Six members projected only one hike this year, although the median expectation remained two hikes this year. U.S. stocks gave up gains towards the end of the trading day.
Yellen said at her press conference Wednesday that Brexit concerns were a factor in the central bank's latest monetary policy decision.
"It was fair to say it was one of the factors that factored into today's decision," she said at a news conference after the Fed kept interest rates unchanged, as was widely expected.
Meanwhile, the Bank of England said next week's Brexit vote could harm the global economy, minutes from their June 15 meeting showed. "Through financial market and confidence channels, there are also risks of adverse spill-overs to the global economy," the minutes said, according to Reuters. The bank also kept interest rates unchanged.
In Japan overnight, the central bank held rates steady Thursday, sending the yen sharply higher and sparking speculation on whether policymakers would intervene to halt the currency's rise. By the end of the trading day, the Nikkei 225 closed down 485.44 points, or 3.05 percent.
The yen also hit its strongest levels against the dollar in almost two years, at 103.55 yen.
Also pressuring U.S. futures were oil prices, which fell more than 1.5 percent to trade near $47.20 a barrel.