A United Airlines flight from San Francisco to Rome had to be diverted Sunday after an unexplained mechanical issue, making an emergency landing eight hours into the flight.

United Flight 507 from SFO to Rome took off at 5:03 pm Sunday and experienced some sort of mechanical issue while flying over the Atlantic, about eight hours into the flight. As the Chronicle first reported, the Boeing 777-200ER jet suddenly descended to 21,000 feet and issued an international distress signal.

The flight was then diverted to Iceland, where it landed safely at Keflavik International Airport near Reykjavík at 8 am local time.

All 275 passengers and 14 crew members on board were able to safely deplane, and the remainder of the flight was canceled.

"We scheduled a new flight to take our customers to Rome on Monday and have provided them with hotel accommodations," United Airlines said in a statement.

The airline still has not provided details about the source of the mechanical issue that led to the diversion.

The Chronicle notes that the plane involved in the incident began service in 1999, so it has been flying for 26 years.

This incident comes on the heels of multiple emergency diversions in recent months involving United's fleet. Two United flights out of SFO had to be diverted for maintenance issues earlier this month, one headed to Shanghai and the other to Sydney.

A flight out of LAX to Shanghai had to divert to SFO in March because the pilot had forgotten his passport.

Photo by David Syphers