Shares of Apple, the most valuable US company by market capitalization, jumped 8.2 per cent to $US567.77, the biggest gain since August, a day after the company posted revenue that far outpaced expectations. Apple also approved another $US30 billion stock-buyback plan, raised its dividend and authorized a seven-for-one stock split.

The S&P information technology index rose 1.1 per cent and was the best-performing sector of the day.

The three major US stock indexes had opened sharply higher, with the Nasdaq initially climbing more than 1 per cent before turning negative in the first half-hour of trading. Wall Street bounced off session lows with gains again concentrated in the Nasdaq, but stocks finished the day well off earlier highs.

Much of the volatility was driven by comments from Russian Defense Minister Sergei Shoigu, who said Russia started military drills near the border with Ukraine. Ukrainian forces killed up to five pro-Moscow rebels as they closed in on the separatists' military stronghold in the east.

"The market is biased - from all of the good news we've heard on stocks recently - to go higher," said Kim Forrest, senior equity research analyst at Fort Pitt Capital Group.

"However, there are hot spots in the world that keep flaring up like a campfire that won't go out, and the market needs to deal with that."

Caterpillar rose 1.8 per cent to $US105.28 after the company reported better-than-expected earnings and raised its full-year profit outlook.

Limiting the Dow's advance were Verizon Communications and 3M, whose shares fell after the companies' results missed expectations. Verizon dropped 2.4 per cent to $US46.28, while 3M fell 1 per cent to $US136.65.

The S&P telecom index lost 1.7 per cent. The index has dropped 3.8 per cent over the past two days, its worst two-session performance since June.

The Dow Jones industrial average remained unchanged to close at 16,501.65. The S&P 500 gained 3.22 points or 0.17 per cent, to 1,878.61. The Nasdaq Composite added 21.372 points or 0.52 per cent, to end at 4,148.338.

After the close, Microsoft shares rose 2.4 per cent to $US40.40 after the world's largest software company posted third-quarter results.

Amazon.com advanced 0.5 per cent to $US338.89 in extended-hours trading after the online retailer reported a 32 per cent jump in first-quarter net income and a 23 per cent rise in revenue.

Facebook reported a 72 per cent jump in first-quarter revenue, lifted by its mobile advertising business, after the closing bell on Wednesday. The stock darted between positive and negative territory during the session before closing down 0.8 per cent at $US60.87.

Profits are seen rising 2.9 per cent this quarter, down from the 6.5 per cent growth rate estimated at the start of the year, but above the low of 0.6 per cent seen last week, according to Thomson Reuters data.

Zimmer agreed to buy Biomet in a deal valued at about $US13.35 billion to broaden its portfolio of products that treat bone and joint-related disorders. Zimmer's stock surged 11.5 per cent to $US101.97.

A number of cloud-computing stocks, which have struggled lately but rose last week, opened higher and then quickly sold off. Workday Inc slid 5.3 per cent to $US72.59 while Salesforce.com fell 1.6 per cent to $US54.73.

Volume was light, with about 6.07 billion shares traded on US exchanges, below the 6.59 billion average so far this month, according to data from BATS Global Markets.

Advancing stocks outnumbered declining ones on the New York Stock Exchange by 1,535 to 1,465, while on the Nasdaq, decliners beat advancers by 1,494 to 1,087

Reuters