Brothers aged two and three died after a fire in a bedroom swept through their home, a senior fire officer has said.
The boys named on floral tributes as Jake and Logan were rushed to Huddersfield Royal Infirmary on Saturday afternoon but could not be saved.
Their mother and another brother were able to escape the semi-detached home in Alder Street, Huddersfield, before the crews arrived.
Family members have returned to the scene of the tragedy to leave flowers.
West Yorkshire Police said the cause of the fire was not thought to be suspicious.
Chris Kirby, group manager at West Yorkshire Fire and Rescue Service, said firefighters battled to rescue the boys when they arrived at the scene to be told that two children were still inside.
He said: "They very quickly sent two of their breathing apparatus-wearers inside the property, they extinguished the fire and performed a search of the property and recovered the two boys that were still inside.
"They were then very quickly transported to Huddersfield Royal Infirmary by our colleagues from the paramedics and unfortunately we got confirmation later on that they had lost their lives in this incident.
"So (it's) an absolutely tragic event and our thoughts are with the family affected, clearly it's going to be devastating for them, the local community and there's also an impact on the crews who attended.
"I was speaking to some of them yesterday, they had got tears in their eyes, they were very upset about the scenes that they had witnessed."
Mr Kirby said investigations were ongoing, and the fire was thought to have started in an upstairs bedroom.
He added: "Fire can spread very quickly and within a matter of minutes you can have a significant fire.
"We are in the area this afternoon offering free fire safety checks to people in the community, trying to increase their awareness to stop this kind of incident occurring."
Local Labour councillor Amanda Pinnock, who was leafleting in the area on Saturday, said the community was in shock.
"It's a tragedy," she said. "It's a very close-knit area in Fartown.
"Because of the area, everybody knows everybody.
"They will be shocked by what has happened.
"I feel like I am in shock."
She urged people to get a smoke alarm and to check them regularly.
"The fire service will come and fit them for free," she said. "It's crucial people get them."