US stocks extend gains as Brexit vote dominates markets

US stocks closed higher for a second session on Tuesday as markets focus remains on the UK vote on membership in the European Union, with latest polls showing the remain vote slightly ahead.

The Dow Jones Industrial Average rose 23 points, or 0.14%, to finish at 17,827. The S&P 500 rose 6 points, or 0.27%, to close at 2,088. The Nasdaq Composite climbed 7 points, or 0.14% to close at 4,843.

Fed Chair, Janet Yellen testified before the US Congress and said it is unclear what effect a vote to leave the EU would have, but noted it could lead to volatility in world markets "that would negatively affect financial conditions and the U.S. economy."