LOS ANGELES, June 14 (Reuters) - Six people associated with a chain of medical marijuana shops in southern California were arrested on Thursday as federal authorities continued efforts to rein in commercial trade in the drug.

California voters in 1996 made the state the first in the nation to allow medical marijuana, but the possession or sale of cannabis remains illegal under federal law.

U.S. Attorneys for each of California's federal districts announced a crackdown on commercial trade in medical marijuana in October. They outlined a range of actions, including warnings to landlords and civil forfeiture lawsuits.

The U.S. Attorney for the district that includes Los Angeles and surrounding areas also announced criminal charges against several people associated with a medical marijuana business.

The arrests made on Thursday are the latest since then in the district, said Thom Mrozek, a spokesman for the U.S. Attorney for the Central District of California, which includes Los Angeles and surrounding areas.

The six arrested were associated with G3 Holistic Inc, which has three retail centers in the Inland Empire region of California, according to an indictment against the individuals. The Inland Empire is located in the counties of Riverside and San Bernardino, east of Los Angeles.

Aaron Sandusky, John Leslie Nuckolls II, Keith Sandusky, Paul Brownbridge, Richard Kirchnavy and Brandon Gustafson were charged with conspiracy to manufacture and possess marijuana and maintaining a drug-involved premises, and they face other drug-related charges as well. Aaron Sandusky is the founder and owner of G3, prosecutors said.

All six defendants made an initial appearance on Thursday in federal court. None of them was asked to enter a plea.

"We've done a relatively small number of these cases over the years," Mrozek said. "There have been a series of warnings that have been issued to Mr. Sandusky and the other operators, and they just have failed to comply."

Attorneys for the six men could not be reached.

Ellen Komp, deputy director of California NORML, a group working to reform marijuana laws, criticized the arrests.

"It's time to bring the marijuana market into the light of day, where it can be taxed and regulated," she said.

While arrests of medical marijuana operators have been relatively rare in California, in Montana, medical marijuana dispensaries were shut down as a result of a 2011 state law that revised rules for the cannabis trade, combined with raids and prosecution by federal authorities.

Seventeen states and the District of Columbia have approved medical marijuana. (Reporting by Alex Dobuzinskis; Editing by David Brunnstrom)

Here are states that allow medical marijuana:
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  • Alaska

    Legalized for medical use. <a href="http://www.flickr.com/photos/alanasise/6847095796/" target="_hplink">Flickr: alana sise</a>

  • Arizona

    Legalized for medical use. <a href="http://www.flickr.com/photos/billward/110338154/" target="_hplink">Flickr: Bill Ward's Brickpile</a>

  • California

    Legalized for medical use. <a href="http://www.flickr.com/photos/gerbache/2260207829/" target="_hplink">Flickr: gerbache</a>

  • Colorado

    Also legalized possession by non-medical users. <a href="http://www.flickr.com/photos/dok1/520845832/" target="_hplink">Flickr: dok1</a>

  • Connecticut

    Legalized for medical use.

  • District Of Columbia

    Legalized for medical use. <a href="http://www.flickr.com/photos/bigberto/2770838680/" target="_hplink">Flickr: ~MVI~ (off to coron)</a>

  • Delaware

    Legalized for medical use. Flickr: <a href="http://www.flickr.com/photos/dougtone/7749689644/">Doug Kerr</a>

  • Hawaii

    Legalized for medical use. <a href="http://www.flickr.com/photos/ricardo_mangual/6006230817/" target="_hplink">Flickr: Ricymar Fine Art Photography</a>

  • Illinois

    Legalized for medical use.

  • Maine

    Legalized for medical use. <a href="www.flickr.com/photos/indywriter/2683524563/" target="_hplink">Flickr: indywriter</a>

  • Maryland

    Legalized for medical use.

  • Massachusetts

    Passed ballot initiative for legalized medical marijuana in 2012.

  • Michigan

    Legalized for medical use. <a href="http://www.flickr.com/photos/kayoticblue/213316452/" target="_hplink">Flickr: ckay</a>

  • Montana

    Legalized for medical use. <a href="http://www.flickr.com/photos/csbarnhill/2633187564/" target="_hplink">Flickr: csbarnhill</a>

  • Nevada

    Legalized for medical use. <a href="http://www.flickr.com/photos/http2007/4699361533/" target="_hplink">Flickr: http2007</a>

  • New Hampshire

    Legalized for medical use.

  • New Jersey

    Legalized for medical use. <a href="http://www.flickr.com/photos/paulflannery/4021996652/" target="_hplink">Flickr: psflannery</a>

  • New Mexico

    Legalized for medical use. <a href="http://www.flickr.com/photos/everyskyline/3134662783/" target="_hplink">Flickr: michaelwhitney</a>

  • Oregon

    Legalized for medical use. <a href="http://www.flickr.com/photos/d-powell/2099638403/" target="_hplink">Flickr: digging90650</a>

  • Rhode Island

    Legalized for medical use. Also decriminalized possession of less than one ounce.

  • Vermont

    Legalized for medical use. <a href="http://www.flickr.com/photos/bryanalexander/6129117/" target="_hplink">Flickr: BryanAlexander</a>

  • Washington

    Legalized for medical use. Also legalized possession by non-medical users. <a href="http://www.flickr.com/photos/rose_braverman/6924724331/sizes/l/in/photostream/" target="_hplink">Flickr: Rose Braverman</a>