Rhiwbina Hill has been closed between Cardiff and Caerphilly, South Wales, since the accident, believed to have happened at 4.45am. Police have now confirmed that five youths between the ages of 16 and 18 have been taken to hospital after the red Suzuki crashed.
It was the only vehicle involved in the crash, but two of the five occupants were seriously injured.
They are both 18 years old, Wales Online reports.
A 16-year-old boy, a 17-year-old girl and a 17-year-old boy were also injured, South Wales police confirmed.
All five young people are being treated at University Hospital Wales, in the Heath area of Cardiff.
Police would now like to hear from anyone who witnessed the collision, or from anyone who witnessed the red Suzuki Alto being driven prior to the incident and from anyone who has video/dashcam footage.
The road has been closed for hours since then.
Other roads are congested as a result of the closure, including local routes through the Rhiwbina and Pantmawr districts of north-west Cardiff.
Anyone with information should contact hereciting 2200233494.
it comes after a teenager recently died in a BMW crash on the M60†
Brandon Pryde, known as Brandon Geasley, drove the BMW X4 almost head-on into a police car before finally colliding with a red Vauxhall on the M60 near Junction 27 for Stockport, Greater Manchester. The 66-year-old driver, David Faulkner, was also killed in the impact, which closed off part of the highway for hours.
Police in Wythenshawe, Manchester, had spotted Brandon, 18, driving the vehicle, which had been reported stolen the previous day, and were chasing him.