Latest: A sixth person has been arrested after UK house fire that killed three children

Five people were taken into custody overnight in connection with the murder of Demi Pearson, 14, her eight-year-old brother Brandon and seven-year-old sister Lacie following the blaze in Worsley on Monday.

The children's 35-year-old mother, Michelle Pearson, is in a serious condition in hospital while a fourth sibling, three-year-old Lia, is critically ill.

Three men aged 23, 20 and 19, and a 20-year-old woman, are being questioned on suspicion of murder, while a 24-year-old man is also being questioned on suspicion of assisting an offender.

The 19-year-old man and 20-year-old woman have since been released on bail, the force said.

Speaking at a press conference, Chief Superintendent Wayne Miller said: "I can't imagine what the family are going through."

He added: "My heart breaks for them, it really does."

Mr Miller said the force was "so grateful to members of the public who have come forward and given us information" and said there was a greater police presence on the streets as a result of the attack.

Update 2pm: A family targeted in a suspected arson attack which killed three children had been living under threat, sources have confirmed.

Security measures were put in place at the mid-terrace home in Manchester, including the fitting of a device to the property’s letterbox.

The arrangements, known as target hardening, are generally used to strengthen a building to protect it in the event of an attack.

Five people are being questioned on suspicion of murdering Demi Pearson, 14, her eight-year-old brother Brandon and seven-year-old sister Lacie following the blaze yesterday.

The children’s 35-year-old mother, Michelle Pearson, is in a serious condition in hospital while a fourth sibling, three-year-old Lia, is critically ill.

Greater Manchester Police confirmed there had been "earlier incidents" at the address and said the force had made a self-referral to the Independent Police Complaints Commission.

BREAKING: Police investigating a fatal house fire in #Walkden have made further arrests. https://t.co/tztBxKmBqj

— Greater Manchester Police (@gmpolice) December 11, 2017

It is understood the voluntary referral was in response to police contact with the family less than 24 hours before the fatal incident.

Emergency services were called to the address just before 5am yesterday.

Two 16-year-old boys, Ms Pearson’s son, Kyle, and a friend, who were also in the house, freed themselves before fire crews arrived and put out the blaze.

Kyle reportedly tried to get back into the home to help family members but was beaten back by flames and smoke.

Three men, aged 23, 20 and 18, and a 20-year-old woman have been arrested on suspicion of murder.

A 24-year-old man is also being questioned in custody on suspicion of assisting an offender.

Chief Superintendent Wayne Miller said: "In what is such a heartbreaking set of circumstances, we have been doing our very best to support the family in every way that we can and carry out our inquiries quickly and sensitively.

"We are still appealing for people who witnessed the fire itself or the events leading up to it to get in contact with us and tell us what they know.

"My thanks go to the members of the public who have supported us with our investigation."

Demi was a pupil at Harrop Fold School in Salford, which featured in the Channel 4 documentary Educating Greater Manchester.

Headteacher Drew Povey said: "We are devastated at the tragic loss of life today in our community. Our thoughts and prayers are with the family involved."

The street remains cordoned off, with one end blocked by large steel barriers, as the joint police and fire service investigation continues.