TEXAS - Two young sisters died after their mother left them in a car for more than 15 hours, CNN reported.

The 19-year-old Texan mother, Amanda Hawkins, has been charged with two counts of abandoning or endangering a child, Texas authorities said.

Hawkins and a 16-year-old male friend initially took the children to Peterson Regional Medical Center in Kerrville, but the girls were found to be in "grave condition" and transferred to University Hospital in San Antonio, a news release said.

Brynn Hawkins, one, and Addyson Overgard-Eddy, two, died in hospital at about 5pm on Thursday (June 8).

Hawkins initially told hospital staff that they were at a nearby lake where the girls smelled flowers before collapsing, CNN said.

Police investigators later found that the children were left in a car from Tuesday night until noon on Wednesday while Hawkins and her male friend were with friends at their home.

Police didn't say what caused the deaths, but told local news media KABB that temperatures reached more than 30 deg C during the day on Wednesday.

"She left them in the car - intentionally in the car - while her and the 16-year-old male friend were in the house," Kerr County Sheriff WR Hierholzer said. "They were in the house all night. The male friend for a little bit went to sleep in the car a little while but then went back to the house."

"We had heard that somebody in the house could even hear the kids crying and told her to bring them in the house and she didn't," he said in an interview.

The children were unconscious but Hawkins delayed bringing them to hospital as she did not want to get into trouble, the report said.

Her 16-year-old male friend may also face charges.