"All four people on board exited safely. Subsequently the aircraft caught fire. Emergency services are on the scene."
Yr Aran is a mountain peak on a ridge south of Snowdon, the highest mountain in Wales.
Twitter user Philippa Napper posted a picture showing smoke, writing: "Big fire on peak next to £snowdon Now just black smoke. Accident?"
Huw Price also posted a photograph and wrote: "Think a helicopter has crash-landed on top of mountain across from Snowdon. Huge fire, can see the flames from here."
A Welsh Ambulance Service spokesman said: "We were called at about 1.50pm to reports of an incident in the Snowdonia area.
"The Wales Air Ambulance has been sent to respond and is currently in attendance at the scene."
According to the RAF website, the Griffin HT1 is used as an advanced flying-training helicopter at the Defence Helicopter Flying School at RAF Shawbury near Shrewsbury and the Search and Rescue Training Unit at RAF Valley on the island of Anglesey.
As well as pilot training, the Griffin is used for crewman training - "a very important and integral part of multi-crew operations", the website says.
It adds that it is especially useful for procedural instrument-flying training and practising "complex emergency-handling techniques".
Source : www.telegraph.co.uk/