The bus carrying 34 people lost control on a toll road and struck a concrete barrier before rolling onto its side, authorities say.

Rescue team carry a survivor of a deadly bus crash on a stretcher, in Semarang, Central Java, Indonesia, on Monday.Semarang SAR Office / AP

A passenger bus crash killed at least 16 people on Indonesia`s main island of Java just after midnight Monday, officials said.

The bus carrying 34 people lost control on a toll road and struck a concrete barrier before rolling onto its side, said Budiono, a search and rescue agency chief who goes by single name like many Indonesians.

The inter-province bus was traveling from the capital Jakarta to the country`s ancient royal city of Yogyakarta when it overturned while entering a curved exit ramp at the Krapyak toll way in Central Java`s Semarang city, he said.

"The forceful impact threw several passengers and left them trapped against the bus body," Budiono said.

Police and rescue teams arrived about 40 minutes after the accident and recovered the bodies of six passengers who died at the scene. Another 10 people died on the way to a hospital or while being treated, Budiono said.

The 18 victims being treated at two nearby hospitals included five people in critical condition and 13 in serious condition, he said.

Television news reports showed the yellow bus overturned on its side and surrounded by National Search and Rescue Agency personnel, police and passersby as ambulances transported victims and the dead away from the accident scene.

Witnesses told authorities the bus was traveling at high speed before the driver lost control, Central Java Police Chief Ribut Hari Wibowo said at Dr. Karyadi General Hospital in Semarang where the bodies were being identified.

The driver was a substitute who sustained serious injuries but was able to communicate while under medical care, he said.

"We are still investigating the cause of the crash and questioning the injured substitute driver," Wibowo said, adding that police planned to test the driver for prohibited substances including drugs.