A three-year-old boy and seven-year-old girl have died following a fire at a house in Eynesbury, Cambridgeshire Police have said.
The blaze broke out at a three-storey property in Buttercup Avenue at around 7am on Thursday.
Both children were pronounced dead at the scene.
Their 35-year-old mother was taken to hospital with life-changing injuries, while her 46-year-old male partner suffered minor injuries.
The force said officers are working with fire investigators to establish the cause.
Emergency services remain at the scene and tributes have begun to pile up on a verge near the house.
Windows on the terraced home's second and third floors are completely burnt-out, with the rooms inside blackened and guttering melted away.
Neighbour Charles Cooper, 30, described how "the flames went up fairly swiftly".
"By the time my wife and I woke up, the firefighters had already arrived," he said.
"It took a good three or four hours before the smoke abated. The flames were coming out of the top window."
He said he did not know the family well enough to speak to, but said "we would give them a wave".
"I've seen the children playing in the garden," he said.
Neighbour Peter Kellythorn, 40, said there was a smell "like something might be smouldering" when he awoke on Thursday.
"I got dressed, came outside and there was smoke billowing out from the back window," he said.
He added that a satellite dish on the house appeared to have melted, and said: "The heat - it doesn't bear thinking about.
"It's awful. We hoped everyone had got out. They're fairly new houses and they're all fitted with fire alarms and things. It's just awful."
A GoFundMe page set up by a neighbour had reached almost £2,500 by mid-morning on Friday - far exceeding its £1,000 target.
