The Federal Reserve left interest rates unchanged on Wednesday, but kept the door open to a hike in June while showing little sign it was in a hurry to tighten monetary policy amid an apparent slowdown in the U.S. economy.

It's been hard recently to make sense of the overall US economy because of the conflicting signals from March's data. Growth slowed in March, for example, but employment picked up. Household income increased but spending remained sluggish. Analysts believe such a mixed bag bugs Fed officials too.

The Fed's decision to keep rates unchanged was in line with market expectations. Prices for U.S. equities edged up after the announcement, while the dollar was little changed against a basket of currencies.

Prices for longer-dated U.S. Treasuries rallied. Fed policymakers in March forecast two hikes this year. Most traders kept their bets that the first rate hike of 2016 would come in September. But some argue that it might happen sooner, at the next Fed meeting in June.

"Well, it was a little on the dove-ish side. If you look at just that topline assessment, but the Fed also backed away from the language that was really highlighting their concern for international events.

So, they removed the international risk language and replaced it instead with a promise to continue to monitor events occurring abroad. So, the Fed is really giving an opportunity for a June rate hike should domestic conditions continue to improve back to a moderate level," said Lindsey Piegza, chief economist of Stifel Nicolaus & Co.

An initial estimate of first-quarter GDP growth is expected later Thursday. Experts say with U.S. interest rates still close to zero, the Fed is concerned it would have to rely on more unconventional policy tools should the economy take a turn for the worse.