The United States has seen steady job gains that have beat expectations and lasted longer than expected, but Federal Reserve chairman Jerome Powell acknowledged on Wednesday that all is not well.

Wage gains have remained sluggish, while Americans lag behind workers in other advanced economies in education and skills, the amount of people in the workforce and the cost of health care, Mr Powell told senators.

And the number of people working more than one job is at "a very, very high level right now," he said.

In two days of testimony to Congress, the Fed chief was upbeat about the economy, which is experiencing a record 11th year of growth, but he noted the chink in an otherwise healthy picture.

Though the unemployment rate is near a 50-year low, a little over 5 per cent of workers have to hold multiple jobs and earnings are rising at a rate of 3 per cent a year, just ahead of inflation, according to government data.

While hourly wage gains have picked up speed, "you would have expected them to move higher than that" with the jobless rate so low, Mr Powell said.

"So it's a bit of a surprise" that wages have not risen more, he said.

The job market is tight but it "isn't showing up as tight in wages".

For people with only a high school education, incomes have "stagnated badly for a long time".

Mr Powell said additional education and training to keep up with technological change boosts earnings and reduces inequality.

"That was the American story for a long, long, time," he said, but other countries "have leapfrogged" the US, doing more to contribute to rising educational attainment, which means more people stay in the workforce.

And despite the historically low unemployment rate of 3.6 per cent, "the African American unemployment rate is roughly twice that of the overall unemployment rate", he said, calling the disparity troubling.

Mr Powell repeated the plea of Fed chiefs before him for Congress to address these issues, which the central bank is not equipped nor authorised to deal with.

"We don't have the ability to operate directly on that other than again, by carrying out our mandate of maximum employment and stable prices," Mr Powell said.

AGENCE FRANCE-PRESSE