The UN Office of the High Commissioner for Human Rights has said women in countries hit by the Zika virus should have access to birth control methods, dropping laws against abortion in some cases.

Speaking in Geneva, spokeswoman Cecile Pouilly said the OHCHR was asking governments in Zika-affected countries in Latin and South America to repeal any policies that break with international standards and restrict access to sexual and reproductive health services, including abortion.

"We are asking those governments to go back and change those laws," she said. "Because how can they ask those women to become pregnant but also not offer them first information that is available, but the possibility to stop their pregnancies if they wish?"

Health officials are investigating whether there is a link between Zika infections in pregnant women and a rare birth defect.

Ms Pouilly said that about a quarter of women in El Salvador had experienced physical or sexual violence in the past year.

"That also shows that many of these pregnancies are out of their control and countries obviously have to take that into account," she said.

Ms Pouilly said that safe abortion services should be provided to the full extent of the law: "The key point is that women should have the choice and (make) informed decisions. Women should be able to have an abortion if they want."

To date, the mosquito-borne virus has spread to more than 20 countries in the Americas.

On Monday, the World Health Organisation declared the explosive spread of Zika to be a global emergency.

Meanwhile, Brazil said it is sending samples related to the Zika outbreak to the US Centres for Disease Control and Prevention, after accusations that it was hoarding data and biological material.

Researchers across the world were reportedly being starved of access to Zika samples and information, with many countries relying on older strains from outbreaks in the Pacific and Africa for tests and research.

The WHO and Brazilian health officials said legal issues were complicating the sharing of data, but Brazil's health ministry has now said it has wrapped up Zika field work with a US team and will be shipping two-thirds of the material gathered to the US.

Authorities in Brazil have vowed that the Rio Olympics will not be cancelled because of the outbreak.

Sports minister George Hilton said the topic "is not in discussion", adding that he "lamented material and opinions in the press" speculating that South America's first Olympics might be called off.

"The Brazilian government is fully committed to ensure that the 2016 Rio games take place in an atmosphere of security and tranquillity," Mr Hilton said.

International Olympic Committee president Thomas Bach said earlier this week he was "very confident" the games would take place.

Mr Hilton said the WHO had not banned travel because of Zika, though pregnant women have been advised not to travel to Zika areas.

He also noted the cooler, drier Brazilian weather in the South American winter when the games open on August 5 will drive down the mosquito count.

Elsewhere, the tiny South Pacific kingdom of Tonga has declared it has a Zika epidemic after five people tested positive for the virus and another 265 are suspected of having it.

Health minister Saia Piukala said the outbreak is the island's first and it is awaiting the results of more blood tests that have been sent overseas.

Dr Piukala said there have not been any reported cases of pregnant women contracting the virus or showing symptoms.