The S&P 500 was within 1.5 percent of its 52-week intraday high, while the Dow was within 2 percent of its 52-week intraday high.
"We're at the upper end of the trading range here and the Fed's going to be raising interest rates," said Peter Boockvar, chief market analyst at The Lindsey Group. "This territory becomes more difficult to sustain (and) you wonder what the catalysts are to get us higher from here."
In U.S. economic news, the private sector ADP payroll report was 173,000 in May, a touch below Reuters expectations for 175,000 jobs. The April report was revised up to 166,000 from 156,000.
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"Everyone's going to be holding their breath for the jobs number tomorrow," said Doug Cote, chief market strategist at Voya Investment Management.
The May non-farm payrolls report is expected Friday morning before the opening bell.
Treasury yields were lower, with the 2-year yield around 0.88 percent and the 10-year yield near 1.80 percent.
The U.S. dollar index was mildly higher, with the euro near $1.115 and the yen near 108.9 yen against the greenback.
Ahead of uncertainties such as the "Brexit" vote and the Federal Reserve's decision on interest rates, "right now I think you've got people who are stuck in neutral and the market is stuck in neutral," said Art Hogan, chief market strategist at Wunderlich Securities.
Dallas Fed President Rob Kaplan said in a Dow Jones report he would likely support a rate rise in June or July, while noting the Fed should raise rates gradually.
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Earlier, Fed Board Governor Daniel Tarullo said Britain's possible exit from the European Union will be a factor in the Fed's decision on rates later this month,
"In the short term it is more a question of the immediate impact on markets," Tarullo said in a Bloomberg TV interview, Reuters reported. "If there are implications for growth over time, to the degree it's a factor (in the June rate decision) it's taking into account what will happen in financial markets and the immediate aftermath of the vote."