The first and deputy first ministers have led expressions of sympathy over the fire which is feared to have claimed seven lives in County Tyrone.
The ministers expressed their deepest sadness
A couple and their five young children lived at the house in Lammy Crescent in Omagh.
First Minister Ian Paisley said: "Words cannot fully express our absolute shock and deepest sadness at this horrendous news and no words of comfort seem adequate."
Deputy First Minister Martin McGuinness said: "We can only imagine the tremendous loss this is to the wider family and community."
Public Safety Minister Michael McGimpsey said it was "a terrible tragedy, and my deepest sympathies are with the friends and relations of this family".
"This fire has ripped the heart out of a family and has devastated the local community," he said.
Ministerial colleague Arlene Foster described the blaze as "distressing and nightmarish".
"It is very distressing to even listen to the reports, never mind look at what has happened in Omagh last night," she told the assembly on Tuesday.
SDLP leader Mark Durkan said he was "deeply saddened to learn of this horrific tragedy".
"This will come as shocking and harrowing news, not only to the people of Omagh, but to families and communities across the country," he said.
Several other assembly members and local councillors also expressed their shock.
The alarm was raised at about 0500 GMT
DUP West Tyrone assembly member Thomas Buchanan said he knew "that the people of Omagh are heartbroken".
"Any death is tragic but for an entire family, from the oldest to the little child, to lose their life in one single act is indescribable," he said.
Alliance Party chief whip Kieran McCarthy it was "one of the most devastating fires that Northern Ireland has ever seen".
"This tragedy will have caused a massive amount grief in the wider community and I am sure that local residents will be rallying around the relatives and comforting those in shock at this most difficult of times."