Two French regions in the Caribbean face an epidemic of the mosquito-borne Zika virus and France's government is sending extra hospital equipment and preparing extra medical staff to combat it.

France's health minister Marisol Touraine told reporters Martinique and French Guiana have had 2,500 potential cases and about 100 confirmed Zika cases since mid-December, including 20 pregnant women and two people suffering a temporary paralysis condition called Guillain-Barre syndrome.

"Our system of health and sanitary alert is fully mobilised," Ms Touraine said. "There are three objectives: to prevent, reinforce monitoring and anticipate."

The World Health Organisation (WHO) declared Zika a global public health emergency on Tuesday after being linked to brain deformities in babies in South America. Several thousand cases of microcephaly have been reported in Brazil since October, although researchers have so far not proven a definitive link to the virus. No vaccine exists for Zika.

A few cases have been reported in Guadeloupe and Saint Martin, also part of the French Caribbean. Nine people have come to mainland France with Zika this year, but Ms Touraine said there is no risk of epidemic on the mainland.

She said the government will expand access to testing to include doctors' offices and recommend condom use in the region, where she plans to make a visit later this month to check on the situation.

Health officials say a person in Texas has become infected with the Zika virus through sexual contact, in the first case of the illness being transmitted within the US.

The virus, which has been linked to birth defects in the Americas, is primarily spread through mosquito bites, but investigators had been exploring the possibility it could be sexually transmitted.

There was a report of a Colorado researcher who picked up the virus in Africa and apparently spread it to his wife back home in 2008, and it was found in one man's semen in Tahiti.

Ms Touraine also recommmended that people returning from affected areas avoid donating sperm or undergoing in vitro fertilisation for a month afterwards.