U.S. stocks posted strong gains ahead of a Federal Reserve announcement that is expected to provide more clarity on coming interest-rate increases—the prospect of which spooked markets this week. 

The S&P 500 added 1.3% in early trading Wednesday. The broad index fell Tuesday and has declined in five of the past six trading days. The tech-focused Nasdaq Composite Index rose 1.8% at the open, while and the Dow Jones Industrial Average gained 0.9%, or 300 points.