Thirteen people, including seven children, died when a fire tore through a converted three-story house in the eastern US city of Philadelphia on Wednesday, officials said.

The blaze, which occurred in the popular museum district of Fairmount, was one of the deadliest in recent memory in America's sixth-most populous metropolis.

"This is without a doubt one of the most tragic days in our city's history, the loss of so many people in such a tragic way," Mayor Jim Kenney told reporters.

Philadelphia Fire Department deputy commissioner Craig Murphy said the fire was the worst he had seen in 35 years on the job.

He told reporters that the toll was "dynamic because there is still an ongoing recovery effort inside," adding that two additional people were rushed to hospital.

Eight people managed to escape from the burning house, Murphy said.

He added it was too early to say what caused the blaze but that his department was investigating.

"It's not necessarily considered suspicious but we have all hands on deck because of the magnitude of this fire," he told reporters.

"We're in the process of investigating this to the highest level that we can. We're incorporating all of our resources."

He said that there were four smoke detectors in the building but none of them had been operating.

The building is owned by Philadelphia's public housing authority. 

Devastating

Officers found "heavy fire" coming from the second floor of the three-story rowhouse at the 800 block of N. 23rd St when they arrived at 6:40 am (1140 GMT), the fire department said

"It took 50 minutes to place the fire under control," it said in a tweet.

About 26 people had been living in the building, eight on the first floor and 18 across the second and third floors, according to Murphy.

It wasn't clear how many people were in the building during the fire.

"Obviously the tragedy happened and we all mourn for it. But we can't make judgment on the number of people in the house," added Kenney.

The Philadelphia Inquirer quoted police as saying that the three-story home had been converted into two apartments.

Neighbors reached by local media said they had been shaken by the fire.

Bill Richards, who lives nearby, told the Inquirer that shortly before 7:00 am he had heard a woman scream "Oh my god, oh my god."

"It's totally devastating and upsetting," he said.

The residential neighborhood is just a few blocks walk from the Philadelphia Museum of Art and Rodin Museum.

The death toll was almost double that of a fire in Philadelphia in 2008 when seven immigrants died after a kerosene heater exploded in a three-story brick complex.

In December 2017, 13 people were killed in a blaze in an apartment building in the Bronx in New York City's deadliest fire in 25 years.