• Ambulance for New Zealand rugby star got lost on way to ground • IOC vice-president says Rio is ‘most difficult’ Olympic Games

Rio 2016 organisers said they would urgently look into a claim by a prominent International Olympic Committee member that an ambulance destined for the injured New Zealand sevens player Sonny Bill Williams got lost for an hour and a half.

John Coates, the Australian IOC vice-president who first raised concerns over Brazil’s readiness for the Games in 2014, said Rio 2016 was the “most difficult” ever organised amid ongoing concern over empty seats in venues, stray bullets and logistical challenges.

Williams sustained a ruptured achilles tendon during the second half of New Zealand’s 14-12 defeat against Japan on Tuesday, but after being removed from the field had to wait for an ambulance, according to Coates.

“It’s still difficult,’’ Coates told the Daily Telegraph in Australia. “Every morning we have a meeting with the IOC president and vice-presidents and each of the federations and Olympic committees and we also meet with the organising committee.

“But transport, there’s still incidents. The Olympic lanes are working well for the athletes but Sonny Bill with the achilles injury, the ambulance didn’t know where to go and so it took an hour and a half. Luckily it was an achillies and not concussion.’’

Coates said it had been a testing Olympics. “It’s been very difficult, the most difficult Games we have ever encountered in terms of political and economic background,” he told the BBC. “Seven years ago when Rio were selected they were on the verge of being a top five GDP [gross domestic product] nation in the world. They’re 74th now and it’s been a struggle.”

In 2014, Coates caused controversy when he said preparations for Rio were behind schedule and the “worst ever”. A spokesman for the 2016 organisers said that he was not aware of the incident but would look into it.

He promised that in the wake of a controversial incident in which the window of a media bus was smashed amid conflicting accounts of whether it was caused by a rock or a bullet, security would be stepped up. “Drivers have been given a refresh and will be given further training. A review of the availability of first aid kits is being carried out. Military have changed their plans and increased their presence in the area,” the spokesman said.

“Everyone on the scene has confirmed again and again that it was a stone, thrown by a teenager or a kid. Every single bus is now escorted by a military vehicle. A report is being made available. Security is our main concern and our main mission. We are going to keep paying a lot of attention to this.”

The spokesman said that a second incident, involving a bullet at the equestrianism centre at Deodoro, had also led to a security review.

Last weekend a bullet was found in the media tent at the venue and on Wednesday a second stray bullet was discovered near the stables.

Deodoro is a military complex but the spokesman said that the second bullet was the result of a shootout between police and criminals in a favela two kilometres away. “We have extra confidence that it will not be repeated. The athletes and the horses are safe and the military has adopted new plans as a result of these new incidents,” he said.

“We’re going to keep people safe and we have every reason to believe the efforts of the army will be successful.”