The world should prepare for a "global epidemic" of microcephaly, a condition which restricts head growth in foetuses, as the Zika virus takes root in new countries, researchers said Friday.

Scientists from Brazil and Britain said they had found additional evidence that Zika is what causes the often debilitating disorder, a link already widely accepted in medical circles.

In a study conducted among newborns in Brazil -- hardest hit by a joint outbreak -- nearly half of 32 infants with microcephaly had traces of Zika virus in their blood or cerebrospinal fluid, the team reported.

None of 62 infants born with normal heads tested positive for Zika in their blood.

This "striking association", the researchers wrote in The Lancet Infectious Diseases journal, led them to "conclude that the microcephaly epidemic is a result of congenital Zika virus infection".

If this is the case, "we should prepare for the epidemic of microcephaly to expand to all countries with current (local) Zika virus transmission and to those countries where transmission of the virus is likely to spread," the team wrote.

"We recommend... that we prepare for a global epidemic of microcephaly and other manifestations of congenital Zika syndrome."

The researchers also proposed adding Zika to a category of congenital infections known to happen before or during birth. The list includes toxoplasmosis, syphilis, rubella, cytomegalovirus, HIV and herpes.

Zika is a virus spread mainly by mosquitoes, but in rare cases via sex.

In most people, including pregnant women, it is benign with mild or no symptoms.

- No cure, no vaccine -

But in an outbreak that started mid-2015, it has been linked to microcephaly and rare, adult-onset neurological problems such as Guillain-Barre Syndrome (GBS), which can result in paralysis and death.

More than 1.5 million people have been infected with Zika, mainly in Brazil, and more than 1,600 babies have been born with microcephaly since last year, according to the World Health Organization (WHO).

There is no cure or vaccine.

The researchers said theirs was the first study to compare children with microcephaly to a "control" sample of healthy children -- two controls for every malformed baby.

Using a control group is a way for scientists to test the impact of a single variable -- in this case Zika infection -- between two groups that are otherwise as similar as possible.

Eighty percent of women who gave birth to babies with microcephaly had been infected by Zika while pregnant, the team found -- compared to 64 percent of mothers who delivered healthy offspring.

This meant a very high percentage of pregnant women overall had been infected in Brazil's Zika epidemic area.

These were preliminary results, the team said, with findings on another 400-plus babies included in the study to follow later.

Interestingly, the team found that not all infants diagnosed with microcephaly had abnormalities show up in brain scans.