Rio de Janeiro's hopes of becoming the first South American host of the Olympics have been boosted.

The progress comes after the city appeared to emerge with the fewest criticisms from the IOC's evaluation of the four contenders for 2016.

Rio, Chicago, Madrid and Tokyo are all hoping to succeed London as host of the summer Games, and the IOC evaluation commission today said there would be major challenges for each city.

The 106 IOC members will vote on the 2016 hosts on October 2, and although the commission's report is unlikely to be decisive it could well influence some of the members.

The report cited a lack of public support of Tokyo's bid, and the fact that a number of venues listed by bid leaders as "existing" would in fact need to be built.

The commission also expressed concerns about Chicago's lack of financial guarantees and about Madrid's proposed administrative structure.

The IOC did raise issues with Rio's bid too, pointing out that it would be the least compact Games among the four contenders, and that major transport improvements would have to be carried out.

In relation to Tokyo, the IOC report says: "Concern is expressed at the relatively low level of public support for a 2016 Games in Tokyo as shown in the IOC's opinion poll.

"During the venue visits, it became apparent to the commission that a number of venues listed as existing would in fact need to be built. This lack of clarity could have communications and legacy implications."

Credit is given to Tokyo for having full financial guarantees already in place.

Chicago, who have been viewed as front-runners, are having to address concerns over a lack of financial guarantees and some transport issues.

"It is felt that the emphasis on major temporary or scaled-down venues increases the element of risk... in regard to the planning, costing and delivery of the venues," adds the IOC's report.

In relation to Madrid, the proposed administration was a big concern.

The report states: "The candidature file and supporting documentation, as well as the administrative structure proposed for a Madrid 2016 Games, did not demonstrate a full understanding of the need for clear delineation of roles and responsibilities, including financial, between different stakeholders to ensure an efficient and timely transition to the [organising committee], or of the management of operations required to implement the Games vision, concept and plans.

"Given the current complexity and magnitude of delivering a major multi-sports event such as the Olympic Games, this could result in organisational and financial challenges."

Rio's challenge with regard to an insufficient number of hotel rooms was also raised - it plans to use six cruise ships as part of visitor accommodation - while "public safety challenges" were also noted with the IOC saying it was "already showing positive results".

The presentations and documentation provided by each candidate also varied. Rio's were of "very high quality", Chicago's "of high quality", Tokyo's documentation were of "high quality", but some presentations "lacked detail and clarity" while Madrid's "varied in quality".

Carlos Nuzman, president of Rio 2016, said: "I am very happy that this report confirms the IOC's approval of our project and vision for the Games and that Rio is considered ready to welcome the Olympic and Paralympic Games to South America for the first time.

"The evaluation commission's visit to Rio was a great success and this has been highlighted in their description of our documentation and presentations as of 'a very high quality'."