GENEVA -- One of the leading critics of the World Health Organization says he was recently invited to sit on the UN health agency's Zika emergency committee, only to have his invitation rescinded when he refused to sign a confidentiality clause.

Last month, Canadian professor Amir Attaran and more than 200 colleagues wrote an open letter to WHO, accusing it of shirking its responsibilities by not considering whether to recommend delaying or cancelling the Rio de Janeiro Olympics. He then received an invitation from WHO Director-General Dr. Margaret Chan to sit on WHO's independent committee of Zika experts.

That committee met Tuesday and announced after its conclusion that there is a "very low risk of further international spread of Zika virus as a result of the Olympic and Paralympic Games." The group reaffirmed its previous advice that there should be no travel restrictions to Brazil. Zika is mostly spread by mosquitoes and can also in rare instances be spread via sex.

Attaran said he had been surprised that WHO invited him to join their expert committee in the first place, "but thought it would be a good opportunity to have a full discussion of the concerns around Zika and the Olympics."

WHO then sent him a number of forms to sign -- including one with a clause that deems the committee deliberations, including his own comments, to be secret. Attaran signed and returned the forms, but crossed out that particular clause.

WHO spokesman Christian Lindmeier said Tuesday that because Attaran did not agree to the standard confidentiality form required of all experts, he was not issued a formal invitation and, thus, there was nothing to rescind. Lindmeier said that WHO was unaware of any previous cases of a potential committee member refusing to agree to keep deliberations secret.

"The confidentiality agreement is to hold confidential deliberations and decisions of the committee. That's very important to allow for open and unbiased discussion," Lindmeier said. "But the transparency is assured afterwards by making all public what needs to be made public."

Attaran said that barring scientists from repeating their own comments made during the emergency committee deliberations was unjustified and that there were no such requirements tied to the initial offer he received to participate.

WHO's emergency committees are convened according to the International Health Regulations, known as the IHR, a binding international treaty that sets out protocols for how to fight outbreaks.

"The director-general of WHO may appoint members of the emergency committee according to the IHR," Attaran said. "She doesn't get the right to gag them."

Lindmeier said the open letter authored by Attaran and others would be part of the background materials considered by the Zika expert meeting.

In the letter, the experts argued that going ahead with the Rio games as planned will speed up the spread of Zika globally and result in the avoidable births of brain-damaged babies.

WHO declared Zika to be a global emergency in February. While the mosquito-transmitted virus is harmless to most people, it has been proven to cause a number of severe birth defects and a rare syndrome that can result in death or temporary paralysis.

Cheng reported from London.