Three children and a woman have died in a fire at a family home, police said.
Officers and firefighters were called to reports of the blaze in Rosamond Street in Bolton at around 9am on Saturday.
A man managed to escape the terraced house but two boys and a girl, all under the age of 13, and the woman were still inside, Greater Manchester Police (GMP) said.
One of the children was pronounced dead at the scene and the woman and two other children died later in hospital.
The man, who an eyewitness describes as "hammering" on the door of the terraced house, is being treated at a hospital and is in a stable condition.
An investigation has begun into the cause of the blaze, which is not thought to be suspicious.
The eyewitness reported seeing two people being given CPR as emergency services tackled the fire.
The woman said she was alerted to a "commotion" at around 9am on Saturday and saw a man banging on a door of one of the terraced houses.
Crime scene investigators were at the scene, in the Daubhill area of the town, she said.
The resident said: "It's terrible, absolutely terrible. I saw them bringing people out. They were doing chest compressions.
"I saw them bring two out and then they put a green sheet up. I don't know if they were bringing fatalities out.
"It was 9am this morning when I heard all this commotion. Somebody - I don't know whether he lived there - there was just a load of hammering, banging on a door.
"I went to the window and saw smoke billowing.
"When I saw him after he had his hands bandaged up and his head. He either lived there and managed to get out or he was trying to get in."
Detective Chief Inspector Chris Bridge, from GMP's Bolton borough, said: "These are utterly heart-breaking circumstances and our thoughts go out to anyone affected by this tragic incident.
"This appears to be a family home and we have been working to inform loved ones of this awful loss.
"I know the community will feel the impact of this and I assure you we are working with our colleagues at GMFRS to get to the bottom of what happened, although we are currently not treating it as suspicious.
"This means there will be officers and fire investigators in the area for some time and I would like to thank residents for their patience so far.
"This happened on a Saturday morning when many people would be up and about and I would appeal to anyone with any information about this incident to please call us."
A GMFRS tweet said: "Our deepest condolences go to the family and the community. We will be in the local area in the coming days reassuring residents."
Greater Manchester Mayor Andy Burnham said: "Dreadful news coming out of Bolton today. My thoughts are with the family, their friends & the whole community."
GMFRS assistant county fire officer Tony Hunter told reporters the man was believed to be the children's father and he had escaped by jumping from a first floor window.
He had tried to get back into the home to rescue his children and their mother but was unable to do so until four firefighters used a specialist tool to break the door down, Mr Hunter added.
Another eyewitness, Aymen Mohamed, said he had been visiting his grandmother when the fire broke out.
The 24-year-old said residents in the street came out on to the street and saw the man, named locally as Zubair, jump from the window.
He said: "Early this morning about 9am there was smoke emerging from the house and noises of shouting.
"Residents of the street gathered and it was seen that a man escaped by jumping out of the window.
"I didn't see the man but it was said he jumped from the back window which faced the backstreet so it was hard to see.
"I knew of the family, but not as friendly.
"I saw the grandmother of the kids crying once it was known they were dead."