A United Airlines passenger plane was forced to return to Sydney Airport on Monday morning after apparently striking a number of birds when taking off.

Emergency service workers were called to the international terminal about 10.50am to meet United Airlines Flight 840, which had taken off at 9.40am.

The plane, bound for Los Angeles, was carrying 308 passengers and crew.

"Unbeknownst to me, we apparently hit birds during takeoff and flight crew spotted bird strike damage to the plane," a passenger wrote on Instagram.