LONDON (AP) — Brazil says it's sending a set of Zika samples to U.S. health authorities following complaints over the South American nation's hoarding of data and biological material related to the disease.

The announcement came hours after The Associated Press revealed that international health officials were frustrated at Brazil's refusal to share enough viral samples and other information to answer the most worrying question about the outbreak: Whether the disease is truly causing a spike in babies born with abnormally small heads.

Brazil's health ministry said in an email statement on Friday that it was "at the disposal " of international researchers studying Zika.

U.S. and U.N. officials have told AP that Brazil has so far probably shared fewer than 20 samples when experts say hundreds or thousands of samples are needed to track the virus' evolution and develop accurate diagnostics and effective drugs and vaccines. Many countries' national laboratories are relying on older strains from outbreaks in the Pacific and Africa, the AP found.

After the story's publication, the World Health Organization sent out a flurry of messages acknowledging that existence of a "data gap."

"Given the complexity of unanswered questions on Zika & (associated) disease, our goal is to encourage all researchers to share their data ASAP," the WHO said on Twitter. "Rapid data sharing is critical during an unfolding health emergency."

In a statement posted to its website on Thursday, Brazil's health ministry said that two-thirds of the material gathered during recent field work with an American team would be shipped to the Centers for Disease Control and Prevention.

"This measure has been submitted to the National Committee of Ethics in Research which approved the project in full," the statement said.

The number and precise nature of the samples was not made explicit. It was also unclear under what conditions the Brazilian viruses were being shared.

"One crucial question is whether the bilateral sharing comes with any strings attached," said Lawrence Gostin, a global health law expert at Georgetown University. In an email, he said countries would likely be reluctant to share samples freely without agreements to ensure they wouldn't be frozen out of any resulting products from their viruses, like drugs and vaccines.

Zika was discovered in a Ugandan forest in 1947; until last year, the virus had never caused serious disease. It has now spread to more than 20 countries.

The AP's revelation that Brazil was not sharing many virus samples came as a surprise to several senior scientists, including officials at WHO and elsewhere. Dr. David Heymann, chair of WHO's Zika emergency committee, said that virus-sharing was not discussed during the hours-long crisis meeting on Monday.

"Virus-sharing was not mentioned as an issue of concern," he said.

When asked about sample-sharing this week, Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases told the AP: "I don't think it's an issue."

Some were dismissive even after the AP's report was published. When asked Thursday about the importance of sharing samples, senior WHO official Dr. Sylvie Briand said that obtaining the latest strains wasn't critical because the mosquito-borne virus had not mutated.

Shortly afterward, WHO said that breaking the logjam over sharing was a top priority.

"Deficiencies (with) existing data-sharing mechanisms (have) brought the question of data access to the forefront of the global health agenda," the U.N. health agency said in a tweet.

Many scientists say having access to current samples is critical to figuring out whether it is mutating into a more transmissible or dangerous form. With so many unknowns about Zika, researchers are keen to sequence the virus to see if it has evolved into a form capable of causing microcephaly, the feared birth defects seen so far in babies born in Brazil and French Polynesia after Zika's arrival.

"The work needs to be done and the quicker it's done, the better," said Ben Neuman, a virologist at Britain's Reading University.

"We basically can't follow Zika without data," he said. "Not sharing is ridiculous, frankly."

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Jamey Keaten in Geneva, Joana Mateus in London and Mike Stobbe in New York contributed to this report.

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Online:

Brazil's announcement: http://bit.ly/1PEEQSI

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Raphael Satter can be reached at: http://raphae.li