Renewed fears of a second U.S. recession sent shares of U.S. airlines tumbling early Tuesday and a global industry group said that the uncertainty over the global economy would hit carriers during a crucial period of the year.

The International Air Transport Association said air travel volume globally is close to normal, but the outlook is much weaker. A sample of international airlines show that average second quarter financial results have declined by 60 percent, the IATA said. And fuel prices, although heading lower, are still up 50 percent from last year.

New aircraft deliveries have increased costs and put pressure on airlines to fill more seats just as "financial market gloom rises."

Airlines generate most of the year's profit in the third quarter thanks to the summer travel season, but "market conditions are now deteriorating," the IATA said.

The industry earns 80 percent of its profits in the second and third quarters combined, with losses typical in the first three months of the year and a traditionally weak fourth quarter.

Airlines earned a combined US$1.8 billion in the second quarter, down 60 percent from last year, the IATA calculated.

Major U.S. airline stocks fell more than 2 percent in early trading. United Continental Holdings Inc., the world's largest airline company, slipped 70 cents, or 3.9 percent, to US$17.37. Shares of Delta Air Lines Inc. fell 19 cents, or 2.6 percent, to US$7.08. The Dow Jones Industrials Average lost nearly 300 points in early trading.

The Dow Jones industrial average fell more than 250 points. The losses come after steep declines in European indexes Monday surrounding concerns that Europe's debt problems could slow growth around the world.

Compounding fears, a report Tuesday on the U.S. service sector is expected to show the fourth straight month of declining growth.

It follows a jobs report Friday that that found that no jobs were added to the economy in August. It was the worst reading on the jobs market since September 2010.