Federal Reserve Chair Janet Yellen said Wednesday she still foresees only "gradual" interest rate increases, as the US economy grows stronger and employment expands.

But question marks remain about how the economic outlook will play out, and possible changes in spending and other policies by President Donald Trump's administration are one "source of uncertainty," Yellen said in her semi-annual testimony to Congress.

Another big question is whether inflation will finally move closer to the Fed's two percent target rate, from the current 1.4 percent.