Some 279 pregnant women in the US and its territories have tested positive for infection with the Zika virus, health authorities say.

Some 157 of the women are in the US and another 122 are in US territories, primarily Puerto Rico, according to the US Centers for Disease Control and Prevention (CDC).

The agency said in a conference call that so far fewer than a dozen of the infected pregnant women it has tracked in the US and Puerto Rico have had miscarriages or babies born with birth defects. 

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

US health officials 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 been seen in people returning from countries where Zika is prevalent, such as in Latin America, or through sexual transmission by travellers.

The CDC’s added that their "top priority for the Zika response is to protect pregnant women and their fetuses".

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 World Health Organisation (WHO) said earlier this week that the Zika virus is likely to spread to Europe by late spring and summer.