The mystery of what caused a sonic boom-like noise over Co Down has deepened after the RAF denied reports a supersonic fighter jet was in Ulster skies at the time.

Residents in north Down were alarmed when they heard what they thought was an earthquake on Tuesday afternoon.

News Letter journalist Lesley Walsh was at her Bangor home just after 3pm.

"It lasted for about three seconds and I felt it right through me," she said. "It shook the decking outside, reverberating right through it.

I was on the phone to a friend a mile away and she heard it too.

"At one point I thought it was a bomb, then maybe an earth tremor.

"It was not like ordinary thunder, it was a palpable noise. It was an absolutely perfect blue sky and I could see nothing overhead."

Police said it was "believed to be the sonic boom of a low-flying aircraft", adding their information came from the Coastguard.

However, the George Best Belfast City Airport could offer no explanation for the incident.

Yesterday, a Coastguard spokesman who also lives in Bangor, said he too heard the sound. "I was in my garden and my dog bolted when we heard the noise," he said. "It was like thunder but there was not a cloud in the sky. It was a strange sound."

He said the PSNI also had reports of the noise from Portavogie and that the RAF in Scotland had said it was a supersonic Typhoon fighter jet.

"RAF Kinloss confirmed a Typhoon had been transiting the area and had now departed on its way," said the Coastguard spokesman.

However, an RAF spokesman at the Ministry of Defence in London said none of their aircraft were in the area at the time and he was at a complete loss to explain how the Coastguard had come by a report to the contrary.

The RAF spokesman said a Nimrod had been flying off the coast but agreed it was not a likely explanation.

Four months ago, callers to BBC Northern Ireland reported strange orange lights in the night sky over Bangor. Air traffic control at Belfast International Airport said it had also received reports, including one from the Coastguard, but there were no records of aircraft in the area.

The Mull of Kintyre is only 25 miles away from the Northern Irish coast and in 2002 the tourist organisation Visit Scotland said Scotland had the highest concentration of UFO sightings of any nation in the world.

In January 1996 the Scotsman newspaper reported concerns that the top-secret USAF aircraft Aurora, which flies at three times the speed of sound, was flying from RAF Machrihanish on the Mull of Kintyre.

In articles spanning several years, the newspaper claimed the stealth spyplane may have been responsible for numerous UFO reports and that its unusual sonic boom was thought to be responsible for earthquakes and avalanches in the Netherlands.

Terry Moseley of the Irish Astronomical Association said that there was an alternative explanation if it was not a plane. It could have been "a fireball exploding up in the atmosphere - perhaps a meteorite or a piece of a comet".

"If that happens above the clouds, you would not see it from the ground," he said.

"That is the most likely explanation if it wasn't a jet."