Flight circling after engine problem

• Witnesses report fire in engine
• United flight was taking off for Berlin
• FBI: Piece of tire flew into Boeing 757 engine

New York (CNN) -- A United Airlines flight was returning Saturday evening to Newark Liberty International Airport after a problem developed with the left engine during takeoff, officials said. Eyewitnesses reported seeing flames spewing from the engine.

A tire blew during takeoff and flew into an engine, said FBI spokeswoman Barbara Woodruff said. The Federal Aviation Administration said it could not confirm that.

Flight 96, a Boeing 757, reported a problem in its left engine after it left New Jersey for Berlin, said FAA spokeswoman Kathleen Bergen.

The engine was operating properly and the plane was burning fuel before landing, according to the FAA.

Eyewitness Keisha Thomas, who was traveling on the New Jersey Turnpike, said she witnessed fireballs near a wing at about 6:25 p.m.

Thomas heard a loud sound, describing it as "pow, pow, pow." The plane was circling the airport, she said.

Djenaba Johnson-Jones, who lives near the airport, said she heard an unusual noise and saw fire, but not smoke, coming from the aircraft's left engine.

Eyewitness Dennis Ostolaza said he heard a "propeller sound" akin to a military helicopter as the airliner gained altitude after takeoff, with "black smoke and fire spitting out of the engine."

CNN's Julia Greenberg contributed to this report.