The U.S. government says it won't challenge the first two states that allowed recreational marijuana use from letting the practice go forward.

Colorado and Washington have been moving cautiously while waiting to see how the federal government would respond. Federal law forbids marijuana use and possession.

But last December, President Barack Obama said it does not make sense for the federal government to go after recreational drug users in a state that has legalized recreational use of small amounts of marijuana. Obama himself has admitted smoking pot when he was younger. Last week, the White House said prosecution of drug traffickers remains an important priority.

In its sweeping policy announcement Thursday, the Justice Department outlined eight priority areas for its enforcement of marijuana laws.

They range from preventing the distribution of marijuana to minors to preventing sales revenue from going to criminal enterprises, gangs and cartels and preventing the diversion of marijuana outside of states where it is legal under state law.

Other priority enforcement areas include preventing state-authorized marijuana activity from being used as a cover for trafficking other illegal drugs, and preventing violence and the use of firearms in the cultivation and distribution of marijuana. The top areas also include preventing drugged driving, preventing growing marijuana on public land and preventing marijuana possession on federal property.

The action, welcomed by supporters of legalization, could set the stage for more states to legalize marijuana. Alaska is scheduled to vote on the question next year, and a few other states plan similar votes in 2016.

Currently, 20 states and Washington, D.C., allow medical marijuana — a more restricted usage.

A Pew Research Center poll in March found that 60 per cent of Americans think the federal government shouldn't enforce federal anti-marijuana laws in states where its use has been approved.

Younger people, who tend to vote more Democratic, are especially prone to that view. But opponents are worried these moves will lead to more use by young people. Colorado and Washington were states that helped re-elect Obama.

"Today's announcement demonstrates the sort of political vision and foresight from the White House we've been seeking for a long time," said Ethan Nadelmann, executive director of the Drug Policy Alliance, a New York-based group. "The White House is basically saying to Washington and Colorado: Proceed with caution."

Dan Riffle of the Marijuana Policy Project, the nation's largest marijuana policy organization, called the policy change "a major and historic step toward ending marijuana prohibition" and "a clear signal that states are free to determine their own policies."

Kevin Sabet, the director of Project Smart Approaches to Marijuana, an anti-legalization group, predicted the new Justice Department policy will accelerate a national discussion about legalization because people will see its harms — including more drugged driving and higher high school dropout rates.