A United Airlines Boeing 777 jet clipped the wingtip of another United plane while it was pushing back from the gate early Tuesday morning at San Francisco International Airport, prompting the cancellation of two trans-Pacific flights that had been carrying more than 500 passengers between them, officials said.

       No one was injured during the mishap, the latest incident to bring scrutiny to commercial aviation safety procedures in the United States.

       The brush between the two planes occurred around 12:35 a.m., according to the Federal Aviation Administration, which said that the planes were maneuvering in an area where air traffic controllers do not communicate with flight crews.

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