US stocks have traded flat despite the excitement from two giant M&A deals, the merger of American Airlines and US Airways, and Berkshire Hathaway and 3G’s takeover of ketchup maker Heinz.

Analysts say markets were partly echoing the slump in Europe on dismal economic growth reports from eurozone countries, as well as Japan.

At Thursday’s close, the Dow Jones Industrial Average was down 9.52 points (0.07 per cent) at 13,973.39.

The broad-based S&P 500 added 1.05 (0.07 per cent) to reach 1,521.38, while the tech-rich Nasdaq Composite gained 1.78 points (0.06 per cent) to 3,198.66.

US Airways shares fell 4.6 per cent after the official announcement of its long-awaited merger with American Airlines, creating the largest US carrier.

American parent AMR Corp, under bankruptcy reorganisation, jumped 57 per cent.

Heinz shares soared nearly 20 per cent to $72.49 after the shock announcement that billionaire Warren Buffett’s investment firm would partner with 3G Capital to buy Heinz in a $US28 billion ($A27.21 billion) deal.

Berkshire Hathaway and 3G said they would offer shareholders $72.50 a share.

Berkshire’s B shares finished up 1.2 per cent.