The Dow Jones industrial average slipped into the red for the year on Monday as stocks extended their slump.
Airlines were among the biggest losers amid concern that capacity growth in the industry may curb profitability. JetBlue Airways, American Airlines and Delta Air Lines were among the carriers that declined.
Stocks have sagged in the past two weeks as investors try to assess if Federal Reserve policymakers will raise their benchmark interest rate later this year for the first time since the recession. A stronger-than-forecast jobs report on Friday suggested that the economy is recovering from its winter slump.
The Fed has kept its benchmark interest rate unchanged at close to zero for more than six years to help support lending and boost the economy. Those low rates have also boosted the stock market in that time, pushing it to record levels.
"The market is suggesting that if the data continue at this pace, the Fed will be more inclined to raise rates in September, rather than waiting," said Quincy Krosby, a market strategist at Prudential Financial.
The Standard & Poor's 500 index dropped 13.55 points, or 0.7 percent, to 2,079.28. The Dow fell 82.91 points, or 0.5 percent, to 17,766.55, giving it a loss of 0.3 percent for 2015.
The NASDAQ composite declined 46.83 points, or 0.9 percent, to 5,021.63.
On Monday, slumped as industry executives met in Miami for the International Air Transportation group's annual general meeting.
American Airlines CEO Doug Parker told Reuters at the meeting Sunday that he was worried that growth in airline capacity could depress profits.
Brokerage Raymond James cut its earnings forecasts for American Airlines, Delta Air Lines and United Continental. The firm said in a note that "softer" economic growth would mean that airlines would struggle to raise fares as much as they had previously anticipated during the stronger summer months.
American Airlines was among the leading decliners in the S&P 500, dropping $1.86, or 4.5 percent, to $39.86. Delta Air Lines, United, Southwest Airlines and JetBlue also fell.
The drop for airlines extended a theme of weakness for the transport sector.
The Dow Jones Transportation average is down almost 9 percent this year, a slump that is giving a potentially worrying signal about the economy even as hiring picks up. A drop in transport companies could mean that fewer goods are being transported by boat, plane or rail, suggesting that demand remains weak six years after the end of the recession.
"You do want to see transport stocks gaining," said Prudential's Krosby. "It would lend confirmation (to expectations) that the economy is gaining that crucial momentum that we have been waiting for."
Talks between Greece and its creditors have been deadlocked since late last week, when Greek Prime Minister Alexis Tsipras rejected as unacceptable a proposal made by the three institutions overseeing the country's bailout: the European Central Bank, the International Monetary Fund and the European Commission.
A resolution to the talks is needed by June 30, when Greece's emergency financing program ends. Without fresh funds, Greece is unlikely to be able to repay its debts and could end up crashing out of the euro. Jitters over Greece's financial future have been a cloud over markets in recent days, notably in Germany, where the DAX index is down more than 10 percent from its April peak.
The Wall Street Journal reported that Greece's creditors have suggested extending the country's bailout program until the end of March 2016. The report cited people familiar with negotiations.
Still, some analysts forecast that the current weakness in stocks will prove transitory and stocks will eventually continue to move higher. That's because corporate profits are still strong and interest rates will still be low enough to boost growth.