By Julie Steenhuysen

CHICAGO (Reuters) - Powerful gene editing tools may one day be used on human embryos, eggs and sperm to remove genes that cause inherited diseases, according to a report by U.S. scientists and ethicists released on Tuesday.

The report from the National Academy of Sciences (NAS) and the National Academy of Medicine said scientific advances make gene editing in human reproductive cells "a realistic possibility that deserves serious consideration.”

The statement signals a softening in approach over the use of the technology known as CRISPR-Cas9 that has opened up new frontiers in genetic medicine because of its ability to modify genes quickly and efficiently.

In December 2015, scientists and ethicists at an international meeting held at the NAS in Washington said it would be "irresponsible" to use gene editing technology in human embryos for therapeutic purposes, such as to correct genetic diseases, until safety and efficacy issues are resolved.

Though the technology is still not ready, the latest NAS report says clinical trials for genome editing of the human germline could be permitted, "but only for serious conditions under stringent oversight."

Such editing is not legal in the United States, and other countries have signed a convention prohibiting the practice on concerns it could be used to create so-called designer babies.

CRISPR-Cas9 works as a type of molecular scissors that can selectively trim away unwanted parts of the genome, and replace it with new stretches of DNA.

Genome editing is already being planned for use in clinical trials of people to correct diseases caused by a single gene mutation, such as sickle cell disease. But these therapies affect only the patient.

The concern is over use of the technology in human reproductive cells or early embryos because the changes would be passed along to offspring.

Research using the powerful technique is plowing ahead even as researchers from the University of California and the Broad Institute battle for control over the CRISPR patent.

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Although gene editing of human reproductive cells to correct inherited diseases "must be approached with caution, caution does not mean prohibition," the committee said in a statement.

Sarah Norcross of the Progress Educational Trust, which advocates for people affected by genetic conditions, called the recommendations "sensible and prudent."

But Marcy Darnovsky of the Center for Genetics and Society said they were "unsettling and disappointing," arguing that they "constitute a green light for proceeding with efforts to modify the human germline" - changes that can be passed to future generations.

(Reporting by Julie Steenhuysen; Editing by Marguerita Choy and Andrew Hay)