The federal government on Friday moved to clear the way for the release of genetically engineered mosquitoes into the wild for the first time in the United States, tentatively approving a field test that might help slow the spread of the Zika virus.
The genetically engineered insects, which contain a gene that will kill their offspring, have already shown effectiveness in small tests in Brazil and other countries in suppressing the populations of the mosquitoes that transmit both Zika and dengue fever.
But an application to test the mosquitoes in the Florida Keys had appeared to languish in regulatory review. It has also faced fierce opposition from some residents in the state.
The threat of Zika virus, which is running rampant in Latin America and threatening the United States, spurred the government to speed up its review. On Friday, The Food and Drug Administration said it had determined that the field trial of the mosquitoes was unlikely to cause any harm to people, animals or the environment.
However, a final approval for the trial will not be made until the F.D.A. receives and assesses comments from the public, which is likely to take months.
The mosquitoes, developed by a British company, Oxitec, contain a gene that kills the insect. Male mosquitoes containing the gene are released to mate with wild females. Offspring from such matings die before they reach adulthood, in that way suppressing the population of wild mosquitoes that spread Zika, dengue fever and some other diseases.
The field test, to be conducted by Oxitec and the Florida Keys Mosquito Control District, would take place in Key Haven, a small community outside Key West. The genetically modified mosquitoes would be released up to three times a week for as long as 22 months, to assess whether the population of wild mosquitoes is reduced.
The F.D.A. based its decision largely on a nearly 300-page draft environmental assessment submitted by Oxitec, which is now owned by the American biotechnology company Intrexon. That document was also released on Friday.
Both the company and F.D.A. documents said that the genetically engineered mosquitoes were unlikely to establish themselves in the wild or beyond the Florida Keys area, particularly because they are programmed to die.
They were also unlikely to cause harm to people because the male mosquitoes being released do not bite, they said. While small numbers of genetically engineered females, which could bite, may be released, the extra proteins they contain from the genetic engineering do not look either toxic or allergenic and do not get into the insects’ saliva in detectable amounts.
The F.D.A. also said that if the population of Aedes aegypti mosquitoes, the main type that transmits Zika and dengue, was suppressed, it did not think that the population of other mosquitoes would increase to fill the void. That outcome has been a theoretical concern of entomologists.
The F.D.A. said this was not likely because the trial would be short and because aegypti mosquitoes “occupy a fairly uncontested ecological niche.”
“The consequences of escape, survival and establishment of OX513A in the environment have been extensively studied: Data and information from those studies indicate that there are unlikely to be any adverse effects on nontarget species, including humans,’’ the F.D.A. wrote, using Oxitec’s name for the mosquitoes.
Under federal rules, genetically engineered animals are regulated as animal drugs, giving jurisdiction to the veterinary medicine division of the F.D.A. This was done in order to regulate genetic engineering using existing rules and agencies.
Critics have said that it does not make sense to regulate animals as drugs and that the F.D.A. is not the best agency to evaluate ecological effects. The F.D.A. said it was assisted in its review by the Environmental Protection Agency and the Centers for Disease Control and Prevention.
Most experts agree that traditional methods of controlling mosquitoes — including spraying insecticides and removing standing water where the insects breed — have not been sufficient, at least in Latin America, and new methods must be tried.
However, federal officials and those at the World Health Organization have said that newer techniques, including the Oxitec mosquitoes, need to be tested more extensively and probably cannot be scaled up quickly enough to make a big difference in the near future.
Oxitec argues that its technique is safe because it targets only Aedes aegypti mosquitoes. By contrast, chemical sprays kill many nontarget insects.
But that has not assuaged the opponents of the field trial.
“I don’t want my kids to be guinea pigs,’’ said Mila de Mier, who owns a real estate company in Key West and has three sons. She organized a petition on Change.org that drew more than 160,000 signatures.
Interest in using the mosquitoes arose after the Florida Keys experienced its first locally transmitted cases of dengue fever in decades, in 2009 and 2010. There have been no locally transmitted cases of dengue since then, giving ammunition to opponents of the mosquitoes.
A big question now is whether the threat of Zika will reduce the opposition to the mosquitoes. Judging from Ms. de Mier, it will not.
She said on Friday that even if the technique could reduce the wild mosquito population, it had not been proved to reduce the spread of Zika. She said she and some residents of Key Haven would contemplate filing a lawsuit if the F.D.A. gave the final approval to the field test.
Intrexon, whose stock closed up more than 8 percent on Friday, also wants to deploy the mosquitoes in Puerto Rico, which already has wild mosquitoes spreading Zika virus.
Some lawmakers, saying that Zika posed an emergency, have pressed for faster action, including shortening or suspending the public comment period on the Florida trial.
Dr. Luciana Borio, acting chief scientist of the F.D.A., has said the F.D.A. would not do that given the great public interest in the issue. She also said that the agency could not authorize emergency use of the mosquitoes in Puerto Rico before it won regulatory approval.