The helicopter crew spotted the aircraft earlier this month
A police helicopter crew has spotted what is described as an unusual aircraft in the Vale of Glamorgan.
The helicopter was above St Athan, which is home to an RAF base and close to Cardiff International Airport.
But three-man crew could not capture any images before the object vanished over the Bristol Channel on 8 June.
South Wales Police said the helicopter did not give chase and described the crew as very experienced. The sighting has been reported for investigation.
The helicopter was hovering in the sky at 0040 BST above the base, waiting to land.
A police spokesperson said: "The crew are very experienced and responded in a professional manner in relation to what they saw.
"In today's skies, there are a wide variety of aircrafts which come in a range of different shapes and sizes and in all probability, this sighting has just confirmed that one of these was in the area at the relevant time," the spokesperson added.
A Ministry of Defence spokesperson said it had heard nothing about the incident.
"But it is certainly not advisable for police helicopters to go chasing what they think are UFOs," the spokesperson said.
Cardiff International Airport said air traffic control provided a service to the police helicopter to ensure it had appropriate air space, including allowing it space to return to RAF St Athan to refuel.
Meanwhile, a Cardiff pensioner says he also saw something "peculiar with lots of flashing lights" on the same day.
George Withrington, of St Mellons, said he reported the sighting to police and was given an incident number.
The ex-RAF glider pilot said he was sitting in his garden reading a book on 8 June when he spotted the object at around 1620hrs.
Picture based on Richard Foxhall's sketch for his book
"I was looking at an aeroplane overhead when I spotted this thing was in the corner of my eye.
"It shifted direction very quickly, in the blink of an eye.
"I looked at it for quite a while, i was watching it for at least 10 minutes. It flew off towards the east, towards Newport."
Author Richard Foxhall, who lives in the Nantlle Valley, Gwynedd, has written a book about his experience of seeing a UFO in 1996.
Mr Foxhall has called for the report into the Vale of Glamorgan sighting to be made open to the public.
"The helicopter crew, who are experienced, saw something which was in the air space around Cardiff International Airport and St Athan air base.
"If this was Heathrow there would be an inquiry," he added.
Mr Foxhall said he saw a low-flying object over Tal-y-Mignedd in Snowdonia over the May bank holiday weekend, 12 years ago.
He was returning home at around 2030 BST and saw an object similar to "nothing he had seen before".
Mr Foxhall contacted all local airports, RAF Valley and checked with the authorities and found no flight records had been logged.