BANGKOK/KUALA LUMPUR: Tropical Southeast Asian countries said on Friday they were bracing for the mosquito-borne Zika virus, with Malaysia saying it could "spread quickly" if introduced, but Thailand appeared to be bucking the trend with just a handful of cases a year.

Zika, linked to severe birth defects including babies born with abnormally small heads, is wreaking havoc in Brazil where the government has deployed more than 200,000 troops to eradicate mosquitoes.

The World Health Organization (WHO) said on Thursday that the virus was "spreading explosively" and could infect as many as four million people in the Americas.

No treatment or vaccine is available.

In Thailand, where just one case of Zika has been reported so far this year, the likelihood of Zika spreading was low, officials said, partly because of better access to health care and because Thailand was dealing with a smaller area.

"Thailand is a medium-sized country with a good public health system and easy-to-access medical facilities," Amnuay Gajeena, director-general of the Disease Control Department, told Reuters.

Thailand detected its first Zika case in 2012 and has recorded an average of five cases a year, according to the Ministry of Public Health.

Kriengsak Limkittikul, assistant professor at the Department of Tropical Medicine at Mahidol University in Bangkok, said there was inadequate information about Zika but that it was "only a matter of time" before more cases were reported.

Thailand has confirmed one case of the virus so far this year. Earlier this month, Taiwan reported one case of Zika infection in a man from northern Thailand.

"NO EVIDENCE OF IMMUNITY"

The WHO said on Sunday the rapid spread of Zika in the Americas was due to a lack of immunity among a population that had not been previously exposed to the virus.

Amnuay said there was "no technical evidence" of widespread immunity in Thailand but individuals exposed to the virus would "of course develop" antibodies as with other viruses.

The Zika virus is spread through the Aedes aegypti mosquito - responsible for dengue, yellow fever and other tropical diseases.

The WHO's Western Pacific Region Office in Manila said as long as Aedes mosquitoes circulated in the region "it can be anticipated that the virus will emerge".

Malaysia's Health Ministry said Zika had not yet been detected. "If it is introduced by an infected Malaysian or by a visitor to Malaysia, it could spread quickly," said Lokman Hakim Sulaiman, Malaysia's Health Ministry deputy director-general.

Neighbouring Singapore has not detected any Zika infections but the government said there was a high risk of transmission if cases were imported to Singapore, a regional travel hub.

In the Western Pacific, Zika was first reported in Micronesia in 2007. It was reported in French Polynesia in October 2013, and since then, a number of Pacific Island countries have reported cases, including New Caledonia, Cook Islands, Solomon Islands, Vanuatu, Fiji and Samoa.

In Australia, the foreign ministry's travel advice website said there had been no reported cases of Zika.

The New Zealand Herald reported on Friday that one local man had been admitted to hospital with symptoms linked to the Zika virus. The Ministry of Health said it had received nine Zika notifications this year, the newspaper reported.

All of the travellers had been in the Pacific Islands and eight of them had recovered.