WASHINGTON (Reuters) - The U.S. National Transportation Safety Board said on Monday it is investigating a United Airlines flight on Friday from Lagos, Nigeria to Washington D.C., in which a sudden aircraft movement led to injuries of passengers and cabin crew.

The NTSB said the incident on United Flight 613 occurred as the Boeing 787 was flying in Ivory Coast airspace. Aviation officials in Ivory Coast delegated the investigation to the NTSB, which will issue a preliminary report within 30 days, the agency said.

United said the flight to Washington Dulles in Virginia with 245 passengers, three pilots and eight flight attendants, returned to Lagos after the unexpected aircraft  movement and a technical issue.

The plane landed safely in Lagos and four passengers and two flight attendants were seen at a hospital for minor injuries and have been released.

"We are working with aviation authorities in the U.S. and Nigeria to understand the cause," United said.

(Reporting by David Shepardson; Editing by Chris Reese and Lincoln Feast.)