GENEVA (AFP) - The World Health Organisation's (WHO's) Zika response programme is only 13 per cent funded, "severely" compromising efforts to combat the virus that is increasingly becoming a global threat, the UN agency said on Monday (May 30).

But the significant funding gaps in the US$17.7 million plan are not having a major impact on Brazil's efforts to keep the 2016 Olympic Games in Rio de Janeiro safe, WHO spokesman Nyka Alexander told AFP.

The UN agency last week rejected a call from 150 international doctors to change the timing or location of the Rio Games, with Brazil the country hardest hit by the Zika outbreak.

WHO said that shifting the Games would not substantially alter the risks of Zika spreading globally, but has urged athletes and visitors heading to Rio to take extra precaution against the mosquito-borne virus.

The organisation has advised women who are pregnant or planning to become pregnant to stay away altogether.

The virus can cause birth defects including microcephaly in which babies are born with unusually small heads and brains, as well as a potentially fatal neurological disorder called Guillian-Barre Syndrome.

WHO has provided technical advice to Brazil's health ministry during the Zika crisis, in areas ranging from patient care to mosquito control strategies, Ms Alexander said in an e-mail.

But "the implementation of WHO's advice and guidance is being prioritised ... by other funding, including from the government of Brazil", so the impact of the UN budget gaps is limited with respect to the Olympics, Ms Alexander said.

"That having been said, it is clear that WHO's ability to support the 60 countries currently affected by Zika - and to support other at-risk countries to prepare for the possible arrival of Zika - will continue to be severely compromised if we and other health partners are not sufficiently funded," she added.

Zika cases have by far been most heavily concentrated in the Americas.

The US$8.1 million virus response plan of WHO's regional office, called the Pan American Health Organisation, is only 20 per cent funded, the UN body said in a report.