Three Chinese people were critically injured and three Croatian rowing team members suffered minor injuries when an Olympic bus travelling to the rowing venue collided with a private van on Wednesday.
"It was a major collision," said Australian rowing team doctor Greg Lovell, who was also on the bus.
Australian team officials said the three of the four Chinese people in the van were badly hurt.
"Three of the four people were critically injured and, according to our team doctor, they were very, very seriously injured but I don't know whether or not they died," Australian team spokesman David Pembroke said.
The bus had slammed into a steel pavilion by the roadside, ripping off most of its front on the driver's side, said a Reuters reporter who passed the scene a few minutes after the accident. The van appeared to have been thrown up to 10 metres by the impact with the bus, leaving a trail of debris.
The International Olympic Committee (IOC) said it was aware of the accident.
"The IOC has been informed of a collision that took place around 1400 (0600 GMT) today between a bus transporting athletes to an Olympic rowing venue at Shunyi and a small van," the IOC said in a statement.
"Three Croatian team members - two athletes and a coach - have been treated for slight injuries," it said, adding the athletes were expected to compete on Wednesday as scheduled.
Australian doctor Lovell suffered mild scratches to his knee. Another Australian on the bus had taken one of the passengers out of the back of the van, the Australian team said in a statement.