By Jon Hilsenrath 

WASHINGTON--The Federal Reserve on Wednesday committed to keeping short-term interest rates near zero at least until midyear and set the stage for tough debates in the months ahead about whether to wait even longer.

The Fed "judges that it can be patient in beginning to normalize the stance of monetary policy," officials said, in the key phrase of their January policy statement following a two-day policy meeting that ended Wednesday.

Fed Chairwoman Janet Yellen said in December that the reference to patience means the central bank isn't likely to raise rates at its next two policy meetings. By including it Wednesday, the Fed has taken a rate increase off the table for its March and April policy meetings, but remained open to a move at the June 16-17 meeting.

The Fed has held its benchmark short-term rate near zero since December 2008 in an effort to bolster economic growth and lower unemployment.

Whether June remains a possibility for a rate increase will depend on how the economy and financial markets behave in the weeks and months ahead. The Fed's policy statement acknowledged the complex cross-currents it confronts as it plots a course for later in the year.

"Economic activity has been expanding at a solid pace," the Fed said, in an assessment of how the economy has performed since its December policy meeting.

That marked an upgrade from the "moderate" pace the Fed pointed to last year.

The economy expanded at a pace in excess of 4.5% in the second and third quarters last year and many economists estimate it expanded around a 3% rate in the final three months of the year. The Commerce Department will release new growth numbers Friday. Moreover, the Fed noted "strong job gains" and a lower unemployment rate.

Those judgments pointed to the central bank's confidence about underlying growth and jobs.

But the statement included a subtle new expression of caution about inflation, which has run below the Fed's 2% target for 31 straight months. The Fed puts heavy weight on the expectations of households, businesses and investors for future inflation. It wants those expectations to remain stable.

In its policy statement the central bank noted market-based measures of inflation have "declined substantially in recent months," a signal of some concern about sharply dropping yields in Treasury Inflation Protected Securities markets which could portend continued low inflation.

For now the Fed appears unlikely to be alarmed about this development. Many officials have been putting more weight on survey measures of expected inflation, such as the University of Michigan's monthly survey of households. The policy statement noted that these survey measures "have remained stable."

Ms. Yellen won approval of the statement in a 10-0 vote.

The meeting sets the U.S. central bank on a course for harder decisions at its March 17-18 policy gathering.

Officials at that time will update their forecasts for economic output, inflation, unemployment and interest rates. They also need to decide whether to formally open the door to rate increases in June by removing or altering the patience language from their official statement.

They are facing a challenging economic backdrop. Though the unemployment rate has fallen faster than most Fed officials expected and growth appears to have picked up, the inflation rate could fall further because of declining oil prices.

Meantime, while Fed officials contemplate rate increases later this year, many other global central banks are easing monetary conditions to combat their own problems with low inflation and slow growth. The latest example Wednesday was the Monetary Authority of Singapore, which sought to weaken its currency Wednesday to support its competitiveness with other Asian economies.

The brewing disconnect between the Fed and other central banks could unsettle markets and complicate the U.S. central bank's calculus.

This week completes Ms. Yellen's first year as central bank chief, one in which she followed through on a plan her predecessor put in place to wind down the Fed's third bond-buying program aimed at bolstering the economy after the 2007-09 recession and financial crisis. The question of when to raise rates is more difficult and contentious.

Ms. Yellen's judgments could define her tenure as the Fed's leader. She has taken a more centrist policy view as its chairwoman than she did as vice chairwoman or as president of the San Francisco Fed. In those earlier positions, she advocated for aggressive and sustained easy-money policies. As chairwoman, she has sought a middle ground with colleagues.

Write to Jon Hilsenrath at jon.hilsenrath@wsj.com