Noting the "widespread and accelerating increase" in cases, the report provides several indicators that show how quickly infections are spreading, especially among pregnant women, who face the greatest risk.

As of July 7, Zika had been diagnosed in 5,582 people, including 672 pregnant women, the report said. Positive tests for people with suspected Zika virus infection have increased from 14 percent in February to 64 percent in June. Screening of the blood supply also turned up a 1.8 percent infection rate during the latest week of reporting, which started July 3.

The numbers are likely to be underestimates because 4 out of 5 people infected with Zika don’t have symptoms and don’t seek medical care or are not reported to public health officials. People without symptoms can still pass the virus to mosquitoes that bite and infect other people, and asymptomatic people might unknowingly transmit the virus through sexual contact.

What's noteworthy in the report is the "consistency of evidence that Zika is spreading rapidly in Puerto Rico," said CDC Director Tom Frieden in an interview. "Puerto Rico is in the midst of an epidemic that is spreading silently and rapidly, and every day that passes means more infants at risk."

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About 32,000 women delivered babies in Puerto Rico last year. The recent data show that in June, about 5 percent of asymptomatic pregnant women tested positive for Zika, Frieden said. That's a huge jump from 0.8 percent in February, according to the report.

If 5 percent of asymptomatic pregnant women were infected in the first major month of summer, with several more months of peak mosquito season ahead, "that's three more months of spread, and then we're on track to have 20 to 25 percent of pregnant women infected," Frieden said. "That would mean more than 7,000 affected pregnancies."

Officials and health organizations in the U.S. territory have launched a series of measures to control the spread of mosquitoes and protect pregnant women, from spraying mosquito breeding sites to installing screens in the homes of about 350 pregnant women. Many homes and schools on the island don't have screens.

Authorities have also removed about 1.6 million tires that could act as mosquito breeding sites.

Despite those measures, in June alone, about 322 pregnant women were diagnosed with new infections, the report said, suggesting that much more aggressive measures are needed to control the insects and provide counseling and care for pregnant women.

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Outbreaks of mosquito-borne viruses tend to peak in the late summer and fall in Puerto Rico — hotter months with higher rainfall — raising concern that Zika will continue to spread and increase in the coming months. At the current trend, hundreds to thousands more pregnant women in Puerto Rico could become infected with Zika by the end of the year, the report said.