The helicopter was providing support to another operation
Search and rescue efforts are continuing for a Coast Guard helicopter that went missing off the west coast.
One person has been recovered in a critical condition, and has been brought ashore.
The coastguard say debris was also found spread over a wide area.
Contact was lost with the four crew helicopter at around 1.00am on Tuesday morning.
The Dublin-based R116 was providing safety and communication support, known as Top Cover, to another helicopter.
The Sligo-based Coast Guard R118 had completed an early morning medical evacuation of a crewman requiring urgent medical attention.
He was taken from a UK-registered fishing vessel, approximately 150 miles west of Eagle Island in Mayo.
The major sea search is ongoing off the Mayo coast, approximately six miles west of Blacksod.
Search and rescue helicopters from Sligo and Shannon are on scene, along with the Air Corps.
Ballyglass and Achill all-weather lifeboats are also assisting in the search, along with five local fishing vessels.
The operation is being coordinated by the Coast Guard Rescue Coordination Centre in Malin Head.
The Coast Guard say both helicopters refuelled at Blacksod before going to the scene.
Eugene Clonan is chief of operations with the Coast Guard. He told Newstalk Breakfast earlier it is looking like 'a dark day'.
A spokesperson for the Taoiseach said Enda Kenny – who is currently in the US as part of the St Patrick’s Day programme – is receiving regular briefings on the accident.
"His thoughts are with the families of the brave crew and with the emergency crews currently working in the area," the spokesperson said.
Thoughts with rescue crews, Coastguard staff and families. Privilege to encounter their superb work while serving as Minister for Transport— Leo Varadkar (@campaignforleo) March 14, 2017
While in a statement, Transport Minister Shane Ross also expressed deep concern at the news.
"As the search for the Dublin based helicopter R116 is currently underway I would like to express my sincere support and sympathies for all those involved, particularly those family members who are awaiting news of their loved ones.
"This is an extremely difficult time for all concerned.
"As we await further information I would like to appeal for space to be given to the relevant professionals involved in the search operation to complete their work.
"Once again, I send my utmost support to all those affected."
Sinn Féin spokesperson for fisheries Martin Ferris hopes the rescued crewman recovers and the still missing crew are found safe and well.
He said: "Fishermen go out safe in the knowledge that the brave men and women of the Irish Coast Guard are at hand should the worst happen.
"Their selfless service to those in need has saved countless lives.
"The rescue helicopters are a welcome sight not just on the deck of a boat in trouble, but also to those injured and stranded on Ireland’s mountains.
"This has been a painful year for the Coast Guard having lost their comrade Catriona Lucas last September. This is testament to their bravery and dedication.
"My thoughts are with the families, comrades, and friends of the man recovered and those still to be found."
Well-wishes have also been sent from other emergency services, including the Dublin Fire Brigade.