The altered insects are unlikely to harm humans, animals or the environment, the US Food and Drug Administration (FDA) said in their preliminary report on Friday.

"FDA found that the probability that the release of OX513A male mosquitoes would result in toxic or allergenic effects in humans or other animals is negligible," the authority said.

The "OX513A" variants were designed by British biotech company Oxitec to combat the spread of mosquito-borne infections, including Zika, dengue, chikungunya and West Nile. Oxitec genetically alters the Aedes aegypti mosquito strain with synthetic DNA, shortening their life span and causing the offspring to die before reaching maturity.

Representatives from the US Centers for Disease Control and Prevention and the Environmental Protection Agency also have reviewed the proposal along with the FDA.

WHO backs testing

Oxitec intends to release a number of the insects in the Florida Keys for a field test, following similar probes in Brazil, Panama and the Cayman Islands. The goal of the experiment is to have genetically modified males mate with wild females and thus reduce the mosquito population.

The OX513A mosquitoes have proven to be effective during previous tests. Also, the World Health Organization (WHO) came out in favor of the trials in February, saying that the controversial method might be necessary to wipe out the Zika-carrying insects.

"Time is not on our side here, if you look at how Zika has been spreading in Brazil and other countries," Oxitec CEO Haydn Parry told reporters on Friday. "The sooner we can start the trial, the sooner we show what we can do."

Insects breed faster in warmer spring weather, and some officials also claim that the mosquitoes are growing resistant to insecticides.

Alternative to Oxitec

In their assessment, the FDA found no significant risks that the modified mosquitoes would move beyond the trial area or develop resistance to insecticides.

However, the FDA is obligated to wait for feedback from the public and other stakeholders in the initiative after giving tentative approval for the trials. This process may take months to complete.

Earlier this week, a residents' group called the Florida Keys Environmental Coalition voiced their opposition, arguing that Oxitec's proposal was inadequate and that it would not have proper oversight. The group proposes infecting mosquitoes with a bacteria that curbs the ability to transmit disease.

Anti-GMO activists have also criticized Oxitec's trials, warning that the experiment might open the way for an infestation of a different dangerous mosquito species.

dj/jlw (AP, AFP)