Some 157 pregnant women in the US and another 122 in US territories have tested positive for infection with the Zika virus.

Fewer than a dozen of the infected women in the US and Puerto Rico have had miscarriages or babies born with birth defects, the US Centers for Disease Control and Prevention (CDC) said.

It is the first time the CDC has disclosed the number of Zika-infected pregnant women in the US and its territories.

Health authorities have determined the mosquito-borne virus can cause microcephaly, a birth defect marked by unusually small head size.

The virus can also lead to severe brain abnormalities and developmental problems in babies.

The CDC told reporters it has significantly increased its US testing capacity for Zika, which can also be transmitted through unprotected sex with an infected person.

Virtually all the Zika cases in the continental US so far have been seen in people returning from countries where the virus is prevalent, such as in Latin America, or through sexual transmission by travellers.

It comes after the US swimming team moved a pre-Olympic training camp from Puerto Rico to Atlanta over fears about the Zika virus.

The CDC previously counted only pregnant women who have had a positive blood test for Zika and who are also showing symptoms.

But under a change announced on Friday, the agency said it would now count pregnant women who test positive for Zika in blood tests, regardless of whether they are showing symptoms.

Under the old definition, there were 48 pregnant women with Zika in the 50 US states.

The virus is spread mainly through the bite of a tropical mosquito, called Aedes aegypti.

It can be found in the southern US, but there is no evidence that they have been spreading the virus in the US yet.

So far, a total of 544 Zika cases have been reported among the wider US population.