WASHINGTON — Federal Reserve Chair Janet Yellen said Tuesday that the U.S. economy faces a number of uncertainties that require the Fed to proceed cautiously in raising interest rates. In delivering the Fed’s twice-a-year economic report to Congress, Yellen cited a slowdown in job growth in April and May and said the Fed will be watching carefully to see whether the weaker momentum is temporary or a sign of a bigger problem. Yellen expressed concerns about the global economy, including slower growth in China and the upcoming vote in Britain over leaving the European Union.