A woman stands near a poster explaining about the Zika virus at the Ministry of Health office in Jakarta Thomson Reuters By Kanupriya Kapoor and Glenys Kirana

JAKARTA (Reuters) - Indonesia can not afford to thoroughly check for a possible Zika outbreak, a health ministry official said, as Southeast Asia's most populous country must focus on fighting dengue, a potentially fatal virus also carried by mosquitoes.

The World Health Organisation (WHO) lists Indonesia among Asian countries with possible endemic transmission of, or evidence of, local Zika infections, but authorities in the sprawling nation of 250 million people have yet to report any recent infections.

Both dengue and Zika are spread by the Aedes aegypti mosquito, which is common across Southeast Asia. Neighboring Singapore has over the past week said it detected more than 240 Zika cases while Malaysia reported its first locally transmitted infection on Saturday.

The Philippines confirmed on Monday its first case of Zika this year and said it was "highly likely" it had been locally transmitted.

"At the moment we cannot go out there and test everybody or every suspected case for Zika because it is too costly," Muhamad Subuh, director general for disease prevention and control at Indonesia's health ministry, told Reuters.

"There are other priorities like dengue fever, which is more prevalent and more dangerous, and we have to allocate our resources accordingly."

Like many of its neighbors, Indonesia records thousands of dengue infections a year.

Indonesia is Southeast Asia's biggest economy but the World Bank estimates the government spends 5.7 percent of its gross domestic product on public health, which works out to $99 per person per year, compared with $459 in Malaysia, the region's third largest economy.

Subuh said the ministry was actively monitoring for Zika, but experts said authorities would struggle to identify patients as few hospitals offered Zika testing, and those that did expected patients to pay more than $150 for a test, putting it out of reach of many ordinary Indonesians

"The biggest challenge right now is that we may miss Zika-infected patients because of lack of facilities and testing," said Tedjo Sasmono, a scientist at Jakarta's Eijkman Institute, one of only two facilities in Indonesia capable of diagnosing Zika.

The institute receives no government funding specifically for Zika and relies on funds from private hospitals, he said.

Most people who are infected with Zika have mild symptoms but infections in pregnant women have been shown to cause microcephaly - a severe birth defect in which the head and brain are undersized - as well as other brain abnormalities.

In adults, Zika infections have also been linked to a rare neurological syndrome known as Guillain-Barre, as well as other neurological disorders.

Regional experts say the spread of Zika across Southeast Asia is likely to remain significantly under-reported as health authorities fail to conduct adequate screening.

(Additional reporting by Randy Fabi in Jakarta; Editing by Miral Fahmy, Robert Birsel)