The countdown is on for the Fed's rate decision on Wednesday as policymakers begin their two-day meeting. Although the central bank is expected to keep rates unchanged, Wall Street will listen closely to Chair Jerome Powell's comments on how the economy is holding up.

U.S. stock index futures fell on Monday after President Donald Trump reignited concerns about the effects of a global trade war by introducing new tariffs, while markets awaited the Federal Reserve's monetary policy decision this week.