(Image Credit: Saul Loeb/AFP/Getty Images)

ABC News' John Schriffen reports:

United Airlines says it is conducting a thorough review this morning of three weekend incidents that left hundreds of passengers stranded and some wondering whether the airline's rocky merger with Continental has undermined its operations.

Since their merger two years ago, the two airlines have combined operations this year. Since 2010, the new United has suffered a rash of bad publicity and now has the worst records in the industry for delays.

In June, the last month for which the government released data, United passengers filed nearly 600 complaints. That's five times higher than its nearest competitor and five times higher than before the merger.

"We've seen this huge spike in customer complaints," said ABC News aviation consultant Steve Ganyard. "Now that we're seeing a rash of in-flight emergencies, the FAA is going to want to ask are they having the same problem integrating their maintenance."

United Flight 96 heading from New Jersey's Newark Airport to Berlin was reportedly forced to make an emergency landing Saturday, and witnesses on the ground said they saw fire coming from the Boeing 757's engine as they watched in suspense.

"I ran to the window, looked outside, saw the airplane and saw fire coming out," D'Jenaba Johnson Jones said.

Alex Jackson said, "As it was elevating, I noticed that there was a flame spitting out of the left engine; it was kind of going like pop-pop- pop-pop."

Cellphone video showed the plane circling for two hours and 15 minutes to burn off fuel before all 173 passengers and crew members landed safely back at Newark Airport.

"The crew followed standard procedures in returning to Newark, where the airplane landed safely," United Airlines officials said in a statement Saturday.

United Flight 409 had to return to Newark Sunday morning after the pilot reported smoke in the cockpit. The Boeing 757 was bound for Seattle. And United Flight 1124 bound for Boston had to return to Houston a few hours later because of engine problems.

"The question is, how well is maintenance being done on those airplanes?" Ganyard said. "Is it a coincidence or is there a greater underlying problem that needs to be investigated."

Editor's note: This blog has been corrected to reflect that United has denied reports that United Flight 96 suffered a blown tire.