Montgomery is the highest-profile athlete under suspicion

IAAF general secretary Istvan Gyulai said only a positive dope test would be enough to justify suspending athletes.

Four US athletes have been charged with doping offences by the US Anti Doping Agency - but none has failed a test.

USADA is basing its case on verbal evidence in the federal investigation into San Francisco's Balco laboratory.

"We have to see something," said Gyulai.

"Based on press reports and telephone conversations, we cannot suspend athletes who are heroes - and even if they are not heroes."

If the athletes are declared eligible and they win the trials, then the USOC has no choice but to send them to the Olympics IOC president Jacques Rogge

World 100m record holder Tim Montgomery, Olympic 400m silver medallist Alvin Harrison, former world indoor 200m champion Michelle Collins and former Olympic relay medallist Chrystie Gaines are under suspicion.

Calvin Harrison, twin brother of Alvin, and distance runner Regina Jacobs are also under investigation for alleged drug violations.

Jacobs allegedly tested positive in 2003 for the designer steroid THG, while Harrison faces a two-year ban after a positive test for the stimulant modafinil, his second doping offence.

Gyulai said the IAAF expected all pending US doping cases to be resolved before the start of the Athens Olympics in August.

USA Track & Field said on Tuesday that all athletes facing doping charges were free to compete at the US Olympic trials which begin on Friday in Sacramento, Califorinia.

Gyulai dismissed suggestions by USATF chief executive Craig Masback that the IAAF would bar athletes from Athens if they qualified but still had doping cases pending.

The US Olympic Committee is due to name its Athens team by 21 July.

The International Olympic Committee has adjusted its rules to allow for late substitutions in "exceptional circumstances".

The Olympics begin on 13 August, with the track competition starting on 18 August.

IOC president Jacques Rogge said the final decision on Olympic eligibility lay with the IAAF and the USOC.

"If the athletes are declared eligible and they win the trials, then the USOC, in my humble opinion, will have nothing else to do than to send them," he said.

"If they are not declared eligible, the case is very clear. They cannot be entered by the USOC."