Wall Street stocks closed mixed on Wednesday on signs that US economic activity was cooling, even though investors were careful not to overreact to a slowdown in private sector hiring.

The Dow Jones slipped 0.2 percent to 42,427, while the S&P 500 was flat at 5,970.

The Nasdaq added 0.3 percent to 19,460.

Data from payroll firm ADP showed private sector employment growing by 37,000 jobs in May, sharply below expectations of 115,000 according to a Briefing.com consensus forecast.

But markets shrugged off the figures for now.

"Lower interest rates and the thought of the [Federal Reserve] responding sooner rather than later to signs of a weakening economy have acted as underpinning factors," said analysts from Briefing.com in a note.

On Wednesday, the Fed also noted that "economic activity has declined slightly" in its latest "Beige Book" survey of economic conditions.

It flagged household and business caution due to heightened uncertainty surrounding US President Donald Trump's policies and slower hiring as well. (AFP)