S&P 500 and the Nasdaq have inched to their second consecutive record closing highs on Tuesday as Chinese officials said the deadly coronavirus epidemic could be contained by April.
But the Dow closed flat, and the S&P 500 and the Nasdaq pared their gains after the Federal Trade Commission (FTC) issued orders to large tech companies to provide information on mergers that were too small to report to antitrust regulators. .
Microsoft Corp was the biggest drag on all three major US stock indexes following FTC's demand for data from company. Data was also requested from Alphabet Inc Amazon.com, Apple Inc and Facebook Inc.
The World Health Organisation called the Chinese coronavirus "public enemy number one".
But China's foremost medical adviser on the outbreak said the crisis could be over by April, soothing jitters over the fast-moving epidemic, even as supply chains ruptured and Chinese firms began laying off workers.
"As much as there's a desire to push stock higher there's also an exhaustion," Oliver Pursche, vice chairman and chief market strategist at Bruderman Asset Management in New York.
"It's letting the air out and exhaling. The best thing that could happen for stocks this year is that we go sideways for a while and let markets catch their breath."
Market participants watched closely as US Federal Reserve Chair Jerome Powell began his semiannual economic update before congress.
Powell's remarks reiterated his view that the economy, in its 11th year of expansion, remains "resilient," but that the central bank was closely monitoring potential risks, including the coronavirus.
"Powell addressed the coronavirus and acknowledged that it might be a catalyst for the Fed to take action if warranted," Pursche added.
"It means the Fed is keenly aware of the risks but it also means that the risks are very real."
The Dow Jones Industrial Average was flat, the S&P 500 gained 5.66 points, or 0.17 per cent, to 3,357.75 and the Nasdaq Composite added 10.55 points, or 0.11 per cent, to 9,638.94.
Of the 11 major sectors in the S&P 500, all but communications services, consumer staples and technology closed in positive territory, with real estate showing the largest percentage gain.
T-Mobile shares jumped 11.8 per cent after a federal judge approved its purchase of Sprint, clearing the path for a deal originally valued at $US26 billion ($A39 billion).
Sprint surged 77.5 per cent, while larger rival Verizon Communications Inc slid 2.6 per cent.
Cell tower operators, including SBA Communications Corp , American Tower Corp and Crown Castle International Corp, gained between 4 and 7 per cent on expectations that the deal could increase tower demand.
Under Armour Inc tumbled 16.7 per cent after the sportswear company forecast a surprise drop in 2020 profit.
Advancing issues outnumbered declining ones on the NYSE by a 2.31-to-1 ratio; on Nasdaq, a 1.75-to-1 ratio favoured advancers.
The S&P 500 posted 80 new 52-week highs and 3 new lows; the Nasdaq Composite recorded 152 new highs and 60 new lows.
Volume on US exchanges was 7.33 billion shares, compared with the 7.67 billion average over the last 20 trading days.