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* U.S. and Mexico reach trade deal on Monday
* Nasdaq, S&P open at all-time highs
* Indexes: Dow up 0.07 pct, S&P flat, Nasdaq up 0.10 pct
* Best Buy slides on weak online sales, Q3 profit forecast
* Cosmetics firm Coty, chipmaker Xilinx up on broker upgrades (Updates to early afternoon, adds comments)
By Shreyashi Sanyal
Aug 28 (Reuters) - U.S. stocks treaded water on Tuesday, with the benchmark S&P 500 and the Nasdaq hovering at all-time highs, a day after the United States and Mexico reached an agreement to overhaul the North America Free Trade Agreement (NAFTA).
The two countries agreed on Monday to overhaul the existing trade agreement and U.S. officials expressed optimism a deal could be reached this week.
"The fears of the trade wars have subsided to a certain degree, but I do believe that anything that comes out of this administration needs to be taken with a grain of salt," said Bryan Keller, managing director of Dakota Wealth Management in Palm Beach Gardens, Florida.
The benchmark S&P 500 and the Nasdaq indexes started the session on a strong note to hit fresh all-time highs, as they did on Monday after the trade deal was announced, before pulling back.
"The markets have been in a pretty tight range and I would not characterize this as a sudden change, they are just consolidating at this point of time," said Christopher Harvey, head of equity strategy at Wells Fargo Securities in New York.
"What you will see is a more of a sideways move through the rest of the day," added Harvey.
At 12:44 a.m. ET the Dow Jones Industrial Average was up 18.36 points, or 0.07 percent, at 26,068.00, the S&P 500 was down 0.09 points, or 0.00 percent, at 2,896.65 and the Nasdaq Composite was up 7.85 points, or 0.10 percent, at 8,025.74.
Seven of the 11 major S&P 500 sectors were lower.
The top gainer among the S&P sectors was real estate , with a 0.56 percent rise and the biggest laggard was energy, which was down 0.38 percent.
Best Buy slid 5.5 percent, the top decliner on the S&P, after the electronics retailer's quarterly online sales growth slowed and its current-quarter profit forecast missed estimates.
Coty rose 6.3 percent, the most on the S&P, and Estee Lauder gained 1.2 percent after Morgan Stanley upgraded the shares of the beauty products makers.
Xilinx gained 1.6 percent, after Baird upgraded the chipmaker's shares on an expected ramp-up in 5G-related shipments later this year.
Campbell Soup dipped 1.2 percent after a New York Post report that the soup maker does not plan to sell itself.
Declining issues outnumbered advancers for a 1.41-to-1 ratio on the NYSE and a 1.12-to-1 ratio on the Nasdaq.
The S&P index recorded 39 new 52-week highs and one new low, while the Nasdaq recorded 87 new highs and 15 new lows. (Reporting by Shreyashi Sanyal in Bengaluru; Editing by Shounak Dasgupta)