The White House has pledged to continue to jail those who traffick in marijuana or sell it to minors -- even in two US states where its recreational use is now legal.
At a hearing before the Senate Judiciary Committee, Deputy Attorney General James Cole defended the federal decision not to challenge new laws legalizing marijuana in Colorado and Washington states, approved by voters in November 2012.
Those law clash with federal laws which classify marijuana as a Schedule One controlled substance, on par with heroin.
The US administration has been criticized in some quarters for taking a hands-off position to prosecuting pot users in the two states since the passage of the laws legalizing cannabis.
Coles vowed however that the federal government would continue to go after those seeking to profit from selling marijuana, or those who would seek to traffick the drug to children.
"We're not giving impunity" to drug traffickers, he insisted.
Cole pledged that in addition to blocking cannabis cultivation and distribution, US authorities would work to prevent the export of marijuana to places where the substance is still illegal, "whether the state has legalized it or not."
And he added that Justice Department officials "reserve the right... to challenge the state laws at a later time."
In the US, 21 states have authorized the use of cannabis for medical purposes. Of these, 16 states have decriminalized possession of the drug in small quantities for personal use.
Colorado and Washington went even further, legalizing recreational consumption of the drug. Colorado even allows limited at-home cultivation of the plants.
Drug policy expert Kevin Sabet -- a former official in the administrations of presidents Bill Clinton and George W. Bush -- said legalizing pot was dangerous, because it could lead to the "creation of a Starbucks of marijuana."
"Why would we open the floodgates?" he asked.
But John Urquhart, a sheriff in Washington, denied that that US west coast state was going to become a drug haven.
"What we have in Washington is not the wild, wild West," he said, adding he doesn't see a "conflict" between the federal and state laws.
"We all agree we don't want our children using marijuana. We all agree we don't want impaired drivers. We all agree we don't want to continue enriching criminals," he said.
"Washington's law honors these values by separating consumers from gangs," he said.