The Obama administration is facing criticism over its attempt to straddle the US federal law that makes marijuana illegal and state laws that permit recreational use of the drug.

In the first congressional hearing since the administration announced a new, permissive enforcement policy, law enforcement and drug-prevention groups and their congressional allies see an opportunity to push back. The administration's August 29 announcement allows the two states where recreational marijuana use has been legalised - Colorado and Washington  to go their own way without federal interference as long as they implement strong enforcement systems.

"We are at a precipice," said Kevin Sabet of Smart Approaches to Marijuana, a drug prevention group. "We're about to create Big Marijuana by allowing the commercial production, retail sales and mass advertising of this drug similarly to how we have had Big Tobacco for the last hundred years."

The lead witness at Tuesday's Senate Judiciary Committee hearing was to be Deputy Attorney General James Cole, who signed the guidance putting the new marijuana enforcement standards in place.

Committee Chairman Patrick Leahy, who spent eight years as a prosecutor early in his career, says the Justice Department should focus on prosecuting violent crime and should respect the votes in Colorado and Washington to legalize small amounts of marijuana for personal and medical use.

Iowa Sen. Chuck Grassley, the committee's top Republican and co-chairman of the Senate Caucus on International Narcotics Control, says Attorney General Eric Holder's action was "the wrong message to both law enforcement and violators of federal law."

"When marijuana will be fully legal to buy, diversion of the drug will explode," nine former Drug Enforcement Administration chiefs said in a letter to Holder.

With the door to legalization open in two states, others could follow.

The 20,000-member Marijuana Policy Project says it will support efforts to end marijuana prohibition in 10 more states by 2017. Voters in Oregon and Alaska could consider marijuana legalization measures next year.