An emergency operation is under way after 40 Army cadets, aged between 12 and 17, got into difficulties in the Mourne Mountains.The Northern Ireland Ambulance said 16 of the group are suffering from hypothermia and some had suffered minor injuries.
It is understood the children are all from England. The alarm was raised at about 11:15 BST on Wednesday.
Coastguard, ambulance and helicopter crews were called to the scene.
Medical staff are treating several of the children on the mountain. Weather conditions have been very poor.The coastguard said it is understood that all members of the group have now been accounted for.
The Ambulance Service declared a major incident at 13:21 BST.
Ten accident and emergency crews, a hazardous area response team and Mountain Rescue went to the scene.
The emergency team set up a base at Annalong Community Hall to treat those who were injured.
Ken Johnston, chief press officer with the Ministry of Defence, Northern Ireland, said: "We had a number of young people from England attending an Army Cadet Force camp in Northern Ireland.
"This morning they were in the Mourne Mountains when that atrocious weather swept in. A decision was taken to withdraw as the weather and rain, wind continued."
He described the rescue operation as "remarkable".
"We are exceptionally grateful," he said.
"We are assessing the children, but we understand it is fairly minor injuries, ankle injuries and some cold injuries."
The Coastguard posted on Facebook that as many as 70 people may have got into difficulty.
It said the PSNI and ambulance service contacted the coastguard just before midday on Wednesday requesting assistance after receiving "several reports that a large number of people were in difficulty after being caught out in inclement weather".
Three of its rescue helicopters and three teams went to the scene as well as the Mourne Mountain Rescue Team and police and the UK coastguard.
Colin McGrath, an SDLP MLA, tweeted: "Major emergency incident in the Mournes with helicopter, coastguard, medical and ambulance responding.
"Please God all will be ok."
Police have advised motorists to avoid Head Road in Annalong due to a build-up of traffic. Drivers should seek an alternative route.