U.S. Attorney General Jeff Sessions has rescinded former president Barack Obama's policy that relaxed enforcement of federal marijuana laws in states where it is legal.
Sessions' action will allow federal prosecutors in those states to determine how aggressively to enforce the laws.
Thursday's announcement came three days after retail cannabis stores in California opened for business for the first time, launching what advocates say will eventually be the world's largest market for legal recreational marijuana.
The legalization of marijuana for recreational use in California, seven other states and the District of Columbia may escalate tensions between state and federal drug enforcement officials led by Sessions, a staunch opponent of legalization.
Sessions has compared marijuana to heroin and has blamed it for sharp increases in violence.
The Justice Department considers marijuana a Schedule 1 drug, the same category as heroin, LSD and cocaine. Independent Senator and former Democratic presidential candidate Bernie Sanders responded to Sessions' decision on Twitter, contending that marijuana has been miscategorized.
The use of marijuana for medicinal use is legal in 29 states and the District of Columbia. Democratic Senator Kirsten Gillibrand said Thursday Session's decision could deny patient access to using the substance for medical treatment and disproportionately subject minorities to prosecution.
"This is about public health, and it's about reforming our broken criminal justice system that throws too many minorities in prison for completely nonviolent offenses."
Gillibrand called on her colleagues to support her legislation that aims "to keep the federal government out of the way when doctors and patients decide that medical marijuana is the best treatment for them."
White House spokesperson Sarah Huckabee-Sanders told reporters Thursday "the president believes in enforcing federal law" and Sessions' action gives law enforcers "the tools to take on large-scale distributors."
National polls have shown a majority of Americans now support legalization of marijuana, which has ballooned into a multi-million-dollar industry that helps fund schools and law enforcement agencies.