A United Airlines flight from Rome to Chicago was diverted to Shannon Airport in Ireland on Monday after "a message" was discovered on board, Irish police said.

The message referred to a bomb on the plane, a source with knowledge of the investigation told Reuters.

The 207 passengers and 11 crew members disembarked the Boeing Co 767-330 at Shannon Airport at 2.15pm (1315 GMT), according to Jim Molloy, a representative of Ireland's police force.

The passengers of flight UA971 were in the process of being searched, he said in an emailed statement. He gave no further details about the nature of the threat.

United said in a statement that the landing was due to "a potential security concern" but declined to comment further.

"After assessing the situation our crew made the decision to divert to the nearest available airport," it said. "Additional security screenings will be performed on all customers and baggage, and we will work to get customers on their way to Chicago as quickly as possible."

The Irish Times reported, without citing its source, that a message had been discovered in the plane's toilet claiming that there was a bomb on board.

After passengers disembarked, police began taking handwriting samples in an effort to find out who wrote the message, the newspaper reported. Baggage from the flight was examined as well.