Brazil is not sharing enough Zika samples and disease data to help researchers find out more about the virus.
Scientists are determined to find out whether the virus is, as feared, linked to the increased number of babies born with abnormally small heads in the South American country, UN and US health officials say.
The lack of data is forcing laboratories in the United States and Europe to work with samples from previous outbreaks, and is frustrating efforts to develop diagnostic tests, drugs and vaccines.
Scientists say that having so little to work with is hampering their ability to track the evolution of the virus.
One major problem appears to be Brazilian law. At the moment, it is technically illegal for Brazilian researchers and institutes to share genetic material, including blood samples containing Zika and other viruses.
"It's a very delicate issue, this sharing of samples. Lawyers have to be involved," Dr Marcos Espinal said, director of communicable diseases in the World Health Organization's (WHO) regional office in Washington.
Espinal said he hoped the issue might be resolved after discussions between the US and Brazilian presidents. He said WHO's role was mainly to be a broker to encourage countries to share. When asked whether the estimate of other scientists that Brazil had provided fewer than 20 samples was true, he agreed it probably was.
"There is no way this should not be solved in the foreseeable future," he said. "Waiting is always risky during an emergency."
Last May, as the first cases of Zika in Brazil were emerging, President Dilma Rousseff signed a new law to regulate how researchers use the country's genetic resources. But the regulatory framework hasn't yet been drafted, leaving scientists in legal limbo.
"Until the law is implemented, we're legally prohibited from sending samples abroad," said Paulo Gadelha, president of the Oswaldo Cruz Foundation, Brazil's premier state-run research institute for tropical diseases. "Even if we wanted to send this material abroad, we can't because it's considered a crime."
The ban does not necessarily mean foreign researchers can't access samples. Some were shared with the United States, including tissue samples from two newborns who died and two fetuses recently examined by the Centers for Disease Control and Prevention.
But a US official said that wasn't enough to develop accurate tests for the virus or help determine whether Zika is in fact behind the recent jump in the number of congenital defects. The spike in cases prompted WHO to declare an international emergency on Monday (local time).