BERLIN — Nokia said on Thursday that its third-quarter loss widened to 969 million euros ($1.27 billion), citing sharply declining sales of its smartphones as customers await the introduction of a generation of handsets running a newer version of the Windows operating system.

One analyst said the results had shaken his confidence that Nokia would be able to reverse its decline with Lumia, its line of Windows smartphones. The company said it sold 2.9 million Lumia smartphones in the quarter, down from four million in the second quarter, the first quarterly decline since the line was introduced in late 2011.

“This is probably the first time that I have started to doubt the Nokia comeback story,” said Pete Cunningham, an analyst at Canalys, a research firm in Reading, England. “These numbers were poor and worse than I expected.”

Nokia’s overall loss in the third quarter compared with a loss of 68 million euros in the period a year earlier. Revenue fell by 19 percent, to 7.2 billion euros in the three months through September, driven by declining sales of smartphones and its more basic cellphone line, which makes up the bulk of its sales. Company officials and analysts attributed the slowdown in Lumia sales to consumers waiting for handsets that run Microsoft’s Windows Phone 8 software, which it plans to start selling this year.