In a move marijuana advocates hailed as a historic shift, the Obama administration on Thursday began giving US states leeway to experiment with pot legalization and started by letting Colorado and Washington state carry out new laws permitting recreational use.

The Justice Department said it would refocus marijuana enforcement nationwide by bringing criminal charges only in eight defined areas - such as distribution to minors - and giving breathing room to users, growers and related businesses that have feared prosecution.

The decisions end nearly a year of deliberation inside President Barack Obama's administration about how to react to the growing movement for relaxed US marijuana laws.

Advocates for legalization welcomed the announcement as a major step toward ending what they called "marijuana prohibition".

"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, an advocacy group.

"I must admit, I was expecting a yellow light from the White House," he said in a statement. "But this light looks a lot more green-ish than I had hoped. The White House is basically saying to Washington and Colorado: Proceed with caution."

Marijuana remains illegal and tightly controlled under federal law, even as about 20 states, plus the District of Columbia, allow the use of medical marijuana. Voters in Colorado and Washington state legalized recreational use in groundbreaking ballot measures in November 2012.

Obama had signaled he did not want a new crackdown, telling ABC News in December: "It does not make sense, from a prioritization point of view, for us to focus on recreational drug users in a state that has already said that's legal."

The leeway for the states will go only so far, though, if Colorado, Washington or other states show they are unable to control the drug, the Justice Department said in a statement.

Forty-two percent of Americans aged 12 or older have used marijuana at some point, according to a 2011 survey by the US Substance Abuse and Mental Health Services Administration. Obama has said he used marijuana when he was young.

One opponent of marijuana legalization said his group would redouble efforts to spread word of the negative effects the drug can have on adolescents.