A US pediatricians group says medical marijuana should only be used on children who are severely ill and have no other treatment options.

Medical marijuana should only be used for severely ill kids who have no other treatment option, the most influential US pediatricians group says in a new policy.

Some parents insist medical marijuana has cured their kids' troublesome seizures or led to other improvements, but the American Academy of Pediatrics' policy says rigorous research is needed to verify those claims.

To make it easier to study and develop marijuana-based treatments, the group recommends removing marijuana from the government's most restrictive drug category, which includes heroin, LSD and other narcotics with no accepted medical use, and switching it to the category which includes methadone and oxycodone.

The recommended switch "could help make a big difference in promoting more research," said Dr Seth Ammerman, the policy's lead author and a professor of adolescent medicine at Stanford University.

The academy's qualified support may lead more pediatricians to prescribe medical marijuana, but the group says pediatric use should only be considered "for children with life-limiting or severely debilitating conditions and for whom current therapies are inadequate."

The academy also repeated its previous advice against legalising marijuana for recreational use by adults, suggesting that may enable easier access for kids. It does not address medical marijuana use in adults.

Studies have linked recreational marijuana use in kids with ill effects on health and brain development, including problems with memory, concentration, attention, judgment and reaction time, the group's policy emphasises.

The policy was published online on Monday in Pediatrics. It updates and expands the group's 2004 policy.

Recreational and medical marijuana use is legal for adults in four US states - Alaska, Colorado, Oregon and Washington. Nineteen other states and Washington DC, have laws allowing medical marijuana use only and most allow children to qualify, according to Morgan Fox of the Marijuana Policy Project, a national group that advocates marijuana policy reform and tracks state laws