US President Donald Trump lashed out on Thursday at Jerome Powell after the US central bank continued to hold rates steady again, calling him "too stupid" to be Federal Reserve chairman.

"Jerome 'Too Late' Powell has done it again!" Trump posted on his Truth Social platform, after the Fed decided a day earlier to hold its key lending rate steady for a fifth straight meeting.

In announcing its decision on Wednesday, the Fed cited a moderation in economic activity in the first half of the year and "solid" labour market conditions.

It warned however that "uncertainty about the economic outlook remains elevated", while inflation too is somewhat heightened.

With Trump's sweeping tariffs on US trade partners raising uncertainty about the economic and inflationary outlook, Powell on Wednesday told a press conference that "we're still a ways away from seeing where things settle down".

"He is TOO LATE, and actually, TOO ANGRY, TOO STUPID, & TOO POLITICAL, to have the job of Fed Chair," Trump said.

The president has repeatedly attacked the Fed chief over his rate decisions. (AFP)