The National Highway Traffic Safety Administration will issue a consumer advisory today telling parents that children seated near side air bags are at risk of serious injury or death.

The federal agency, which has been studying the effect of side air bags on children since late last year, decided that parents needed to be warned that the safety devices--which have some benefit for adults--can be harmful to children.

"Tests performed by both NHTSA and auto companies indicate that out of position children are at potential risk of serious or fatal injury to the head, neck or chest from the deployment of side impact air bags, especially if they are near the air bag at the time of deployment. Further, because the data are limited, it is not clear whether properly seated and restrained children incur any benefit from the deployment of side-impact air bags," NHTSA said in a letter sent to car manufacturers advising them of the warning.

The agency told car manufacturers they must be prepared to assure consumers that it is safe for children to sit near side air bags or otherwise ship the cars to dealers with the bags deactivated.

Side-impact air bags, which are installed in about 2.4 million vehicles, are designed to protect occupants in crashes that involve the side of the vehicle. Only a small percentage of side air bags are found in the rear of cars, but these are the ones the agency believes pose the most danger to children who sit too close to them or fall asleep near them. There have been no reported deaths from deployment of side air bags.

This is the first time the agency has given consumers any advice on the safety of side air bags. Some car manufacturers already include warnings in owner's manuals or elsewhere in their cars advising parents not to put children near rear side air bags.

The advisory tells parents who currently own cars with side air bags that they should consult with their manufacturers to see if the devices are safe or ask that they be deactivated.

NHTSA held a public meeting in April at which it expressed concern that tests done by the agency and Transport Canada, that country's car safety regulator, showed that children not seated properly would be harmed by the bags.

Since then, the agency has gotten electronic mail and calls from consumers inquiring about the safety of the bags.

"We decided to get the word out before some kid gets hurt," said Rae Tyson, a spokesman for NHTSA. "The tests just didn't look very good."

Mercedes spokesman Fred Heiler told the Associated Press that the automaker would send a letter to vehicle owners stressing that they should properly restrain children and that there "is a potential for them to be injured if they are out of position or not wearing their seat belts."

But Heiler said the company was concerned that the NHTSA warning would scare parents away from using side air bags that "provide significant protection to adults and children."