NEW YORK (Reuters) - The S&P 500 and Nasdaq declined on Thursday as the possibility of more U.S. tariffs on Chinese imports loomed, while tech stocks stumbled, led by chipmakers and concerns about increased regulation of social media companies.

The public comment period for proposed tariffs on an additional $200 billion worth of Chinese imports ends on Friday at midnight EDT/0400 GMT, and the tariffs could go into effect shortly afterward. China has warned of retaliation if Washington implements any new tariff measures.

On a more positive note, talks between the United States and Canada to renegotiate the North American Free Trade Agreement continued.

The muddled outlook for trade was reflected in the mixed outcome for U.S. stocks on Thursday, investors said. The Dow edged up even as the S&P and Nasdaq fell.

“Investors are waiting for clarity,” said Keith Lerner, chief market strategist at SunTrust Advisory Services in Atlanta. “The next round of tariffs could happen as early as tomorrow.”

Shares of chipmakers and internet companies weighed on the S&P and Nasdaq.

The Philadelphia SE Semiconductor index .SOX slid 2.7 percent after executives from Micron Technology Inc MU.O and KLA-Tencor Corp KLAC.O spoke at a Citi technology conference.

Micron shares sank 9.9 percent and were among the biggest drags on the Nasdaq and the S&P. KLA-Tencor shares tumbled 9.7 percent.

Fallout from the scrutiny of social media companies also continued to batter tech stocks. On Wednesday, executives from Facebook Inc FB.O and Twitter Inc TWTR.N testified before skeptical U.S. lawmakers regarding their measures to combat foreign efforts to influence U.S. politics. The same day, the U.S. Justice Department said it was looking into concerns that social media platforms were "intentionally stifling the free exchange of ideas."

Facebook shares fell 2.8 percent and Twitter shares dropped 5.9 percent. Shares of Snap Inc SNAP.N, the parent company of Snapchat, touched a record low and ended down 3.1 percent.

The Dow Jones Industrial Average .DJI rose 20.88 points, or 0.08 percent, to 25,995.87, the S&P 500 .SPX lost 10.55 points, or 0.37 percent, to 2,878.05 and the Nasdaq Composite .IXIC dropped 72.45 points, or 0.91 percent, to 7,922.73.

Among bright spots, shares of CBS Corp CBS.N rose 3.2 percent after reports that the media company's board was in settlement talks with Chief Executive Les Moonves to negotiate his exit.

Declining issues outnumbered advancing ones on the NYSE by a 1.49-to-1 ratio; on Nasdaq, a 1.89-to-1 ratio favored decliners.

The S&P 500 posted 41 new 52-week highs and 11 new lows; the Nasdaq Composite recorded 98 new highs and 72 new lows.

Volume on U.S. exchanges was 6.66 billion shares, compared to the 6.18 billion average over the last 20 trading days.