This article was reproduced with the kind permission
of the British Broadcasting Corporation
13 November 2007, 11:59 GMT
The fire service was called to the end-terrace house at Lammy Crescent just before 0500 GMT on Tuesday.
Earlier, neighbours put ladders up to first floor bedrooms, but were beaten back by the flames.
Deputy chief fire officer Louis Jones said: "In my 30 years of service, it has to be the largest loss of life in any house fire I have attended."
Firefighters with breathing apparatus were also beaten back by the flames.
It is understood the property was already well-alight when firefighters arrived.
The fire was so intense that the roof collapsed. It is believed the children feared dead range in age from an infant to a 13-year-old.
Parish priest Monsignor Joseph Donnelly said there was "disbelief and shock" in the community.
"It's hard to come to terms with the tragedy, that a family that was alive and well this time yesterday is gone," he said.
Local councillor Paddy McGowan - a former firefighter - said it was "one of the worst tragedies" he had ever seen.
Mr McGowan said the family had lived there for about four years.
Red Cross emergency support volunteers have been at the scene, comforting neighbours and traumatised firefighters.
Local SDLP councillor Dr Josephine Deehan told BBC News the community in Omagh was "stunned by this appalling tragedy".
"The mood here in Omagh is very sombre. Neighbours are standing round in small groups, some of them in tears," she said.
St Conor's Primary School near the house has been closed following the tragedy. Two of the children from the family feared dead went to the school.
The eldest girl, aged 13, went to a secondary school in the town while two other children were under school-age.
Members of the Northern Ireland Assembly expressed sympathy with the people of Omagh.
At the start of proceedings, Speaker William Hay said: "I have to say I have been approached from all sides in the House on the tragedy in Omagh last night.
"Our thoughts and our prayers are with the particular family and their family circle."
First and deputy first ministers Ian Paisley and Martin McGuinness expressed their horror at the incident.
West Tyrone MP Pat Doherty said the scene was "almost beyond words".
"I spoke with the operational commander of the fire service and he said the intensity of the blaze was such that the whole of the inside of the house has collapsed in on itself," he said.