The Coast Guard Rescue Coordination Centre in Malin Head is co-ordinating a search for a Coast Guard helicopter with four crew on board which is missing off the west coast of the country.
A spokesman for the service confirmed that they lost contact with the rescue chopper at approximately 1am.
The Dublin-based crew had been providing top cover for another Coast Guard helicopter that was performing a medical evacuation off the coast of Mayo.
Rescue 116 was returning to base when it fell out of contact, six miles west of Blacksod.
A spokesman confirmed that one crew member has been recovered in a critical condition.
Search and rescue helicopters from Shannon and Sligo are currently searching for the missing craft.
Support is being provided by the Irish Air Corp Casa aeroplane, all-weather lifeboats from Ballyglass and Achill and five local fishing vessels.
In a statement a spokesman for the Coast Guard said: “The Sligo based Coast Guard helicopter R118 completed an early morning medical evacuation (medevac) of a crewman requiring urgent medical attention from a UK registered fishing vessel approximately 150 miles west of Eagle Island in County Mayo.
“Owing to the distance involved safety and communication support, known as Top Cover, was provided by the second Coast Guard helicopter, the Dublin based R116.”
The statement read: “At approximately 1am contact was lost with one of our SAR Helicopters. A major sea search is ongoing off the county Mayo coast approximately six miles west of Blacksod.
“The search operation is being coordinated by the Coast Guard Rescue Coordination Centre in Malin Head. Both helicopters refuelled at Blacksod prior to transiting to the scene.”