BALTIMORE (REUTERS) - Six children were killed on Thursday (Jan 12) after a fire tore through a three-storey house in Baltimore in the night, city fire department officials said.

Their mother was injured in the blaze, along with three more of her nine children, authorities said.

The fire had engulfed the house by the time firefighters arrived on the scene past midnight and its third floor had collapsed onto the second floor shortly afterwards, Baltimore Fire Department spokesman Chief Roman Clark said.

Officials began recovering the bodies of the children, aged between nine months and 11 years old at about 10.30am, he said.

Firefighters had to battle the blaze from outside due to the severity of the flames and destruction. An excavator was used to remove heavy debris before a more detailed search for one of the bodies, Mr Clark said.

"Firefighters are doing what we call 'hand dig' and they are going through it by hand and small tools," he said before the sixth body was found late on Thursday afternoon.

"It's a slow process, we're trying not to disturb any remains."

The cause of the fire was under investigation, Mr Clark said.

The six children who were killed included a baby, a toddler and three-year-old twin girls, according to fire officials.

Maryland Representative Elijah Cummings identified the mother as Ms Katie Malone, a special assistant who had worked in the congressman's office for nearly 11 years, according to a statement from his office.

"My staff is a family and this unimaginable tragedy is shocking and heartbreaking to us all," Mr Cummings said.