The snowfall over Northern Ireland since Thursday has been the worst in 25 years, according to meteorologists.
More than 700 schools have shut and flights have been affected at all three of Northern Ireland's airports.
Motorists are being warned of treacherous conditions and asked only to travel if absolutely necessary.
The Met Office has a weather warning in place until 1800 GMT on Friday amid warnings of more heavy showers.
There are no flights at Belfast International Airport until 1600 GMT. Belfast City Airport reopened for a short time at 1230GMT but flights have been suspended again.
City of Derry Airport will remain closed until Saturday morning.
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".
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.
He 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.
BBC NI weather forecaster Cecilia Daly said snow showers would continue across Northern Ireland for much of Friday morning.
"Another 10-15cms of snow may fall across some parts before the end of the day. There will be drifting in the strong and gusty northerly wind and visibility will be severely reduced," she added.
The Roads Service has said that gritting has been underway since Thursday afternoon.
"Although many of the roads are passable with extreme care, I would stress that there is still the existence of some snow cover despite the extensive work that we have done," said Colin Brown of the Roads Service.
"As soon as the traffic starts to build up on the network, the action of the salt that is underlying that snow should start to free up the roads."
BBC Ireland reporter Andy Martin, reporting from east Belfast, said that the "snow is two feet deep in places with many rural roads completely impassable."
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 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.