Emergency flash warning for heavy snow in NI
An emergency flash warning for heavy snow has been issued for counties Antrim, Down and Londonderry.
The snowfall over NI since Thursday has been the worst in 25 years, meteorologists said.
Flash warnings are only issued by the Met Office in exceptional circumstances.
More than 700 schools have shut and Northern Ireland's airports are closed.
Motorists are being warned of treacherous conditions and asked only to travel if absolutely necessary.
The Met Office said "very heavy snow" was likely "giving additional accumulations of 10-20 cm in many places".
There are no flights at Belfast International Airport for the rest of Friday.
George Best Belfast City Airport had reopened for a short time at 1230GMT but it has now closed for the remainder of Friday.
City of Derry Airport will remain closed until Saturday morning.
Passengers have been advised to check airline websites for further information regarding the status of flights.
BBC weather forecaster Cecilia Daly said that similar snowfalls in 2000 were restricted to eastern counties making the current situation "probably the worst in 25 years".
Earlier, Ciaran Rogan from Translink said bus services were running but some were experiencing extreme difficulties with delays of up to an hour.
He said that while trains were not so badly affected there were still delays of about 10 to 15 minutes with the Belfast to Londonderry line worst hit.
Mr Rogan said minor country routes were now "off the table" with local services redeployed onto the main routes.
There are reports of difficult conditions and heavy traffic on roads in Counties Down, Antrim and Londonderry.
The Northern Ireland Ambulance Service has cancelled ambulance transport for most non-emergency appointments.
Priority is now being given to those patients with appointments for renal and cancer services.
All visits to Maghaberry Prison have been suspended.
The Department of Environment has said there will be no driving tests carried out at any DVA test centres this weekend.
The Roads Service has said that gritting has been under way since Thursday afternoon.
"Since yesterday afternoon when snow started to fall, our 120 gritters, 11 snowblowers and 300 staff have been working around the clock to use all of its resources to salt and keep the main roads open," said Colin Brown of the Roads Service.
"Roads Service salts scheduled routes at three to four times the normal rate during periods of heavy snow and our snow contingency plan has been activated.
"Main routes are passable with care but with more heavy snow predicted across the region, Roads Service is advising motorists to listen to the forecasts and only to travel if necessary during the current severe weather."
Mr Brown said that with temperatures expected to fall to -10C in some areas, motorists should expect icy conditions on roads on Saturday morning.
Regional Development Minister Conor Murphy has advised people wanting to help clear footpaths of snow and ice, that they are unlikely to be held liable if there is an accident.
Belfast City Council suspended bin collections on Friday after lorries experienced difficulties in icy conditions. It said that, weather permitting, the bins would be collected on Monday.
In Londonderry, a gritting lorry driver suffered minor wounds to his cheek on Thursday after snowballs and stones smashed one of the vehicle's windows.
In Glenavy, County Antrim, a gritter came off the road on Thursday night. Roads Service said the driver was tended to at the scene by ambulance personnel and the police. He was shaken but not injured.