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Philadelphia police are seeking car thieves who killed three children.

Three children were killed, and their mother and two other adults injured, when a carjacked vehicle swerved out of control and into a sidewalk church fundraiser in Philadelphia.

At about 11.15am two men with guns hijacked a white Toyota 4Runner SUV, forcing the 45-year-old woman driving into the back seat, Philadelphia Police Captain James Clark told a news conference.

The men made off in the vehicle at high speed and it blew a tire after a mile, skidding across a grassy verge and ploughing through a group of people selling fruit on a sidewalk stall before crashing into a tree.

The three children ‒ aged 7, 10 and 15 ‒ were all killed and their mother, 34, was critically injured, said Clark. An elderly friend of the family was also injured, but is in a stable condition, said police.

The woman who owned the vehicle was also injured, and is in a critical but stable condition.

Police said that she is a real estate agent who had finished a property viewing when the men took over her vehicle.

The two men escaped from the scene on foot. Police are now offering a $100,000 award for information on their whereabouts.

"That's my son ya'll took away from me," said Terence's father. "Whoever you all are, turn yourselves in!" said the father of the dead seven year-old.

The names of the children have not yet been released, as police are informing their families members of their death.

"We now have three children who are gone, and their mother is fighting for her life," Deputy Mayor for Public Safety Everett Gillison told a news conference. "This is a tragedy for our city," he said.

Police pledged to find the men, and are examining CCTV footage of the scene of the carjacking and the crash.

"I've been in the homicide unit for six years and this is one of the saddest cases I've come across," Clark said. "You have three innocent children who had the rest of their lives in front of them that were taken in this fashion.

"It's very sad. It's very tragic and we will find the individuals that are responsible."