US stocks turned negative after the International Monetary Fund warned Greece may need a further large debt write-off, and weak US jobs data dampened the economic outlook. Six of the 10 major S&P sectors were in the red by midday, after opening positive.
IMF's warning comes as Greece readies for a Sunday referendum on an international bailout deal that Prime Minister Alexis Tsipras has urged voters to reject.
The fund said that even if Greek policies came back on track, loans made by Europe "will need to be extended significantly" and that the country would need further concessional financing.
"The market seems to be paying more attention to what may happen with Greece as well as whether or not wage growth will pick up," said Omar Aguilar, chief investment officer of equities at Charles Schwab Investment Management in San Francisco.
Average hourly earnings were unchanged in June, taking the year-on-year increase to a paltry 2.0 percent. Nonfarm payrolls increased 223,000 last month, below the 230,000 that economists polled by Reuters had expected.
At 12:04 am EDT (1604 GMT) the Dow Jones industrial average was down 21.15 points, or 0.12 percent, at 17,736.76, the S&P 500 was down 1.89 points, or 0.09 percent, at 2,075.53 and the Nasdaq Composite was down 13.37 points, or 0.27 percent, at 4,999.75.
BP's US-listed shares rose 4.8 percent to $41.16 after the company agreed to settle claims from the Gulf of Mexico oil spill for $18.7 billion.