By Keith Coffman

DENVER (Reuters) - The use of recreational marijuana is now legal in Colorado, but if a proposed ordinance becomes law in the state's largest city, pot smokers could face jail time and fines if smoke wafts onto a neighbor's property.

A measure under consideration by the Denver City Council would impose up to $999 in fines and a maximum one-year jail sentence for anyone caught smoking marijuana in city parks or other public venues.

But as written, the law would extend the same criminal sanctions to offenders on private property.

"The term 'openly' means occurring in a manner that is unconcealed, undisguised, is obvious, and is observable, perceptible through sight or smell to the public, or to persons on neighboring properties," the draft ordinance reads.

Last fall, Colorado and Washington became the first U.S. states to legalize the possession and use of small amounts of pot for recreational purposes.

Marijuana is classified as an illegal narcotic under federal law. But the U.S. Justice Department has said federal law enforcement will not target users in the two states if they are in compliance with their respective state's laws.

Colorado lawmakers have crafted statewide rules governing the retail sales of cannabis, but the open use of marijuana is missing under the regulations, said Amber Miller, spokeswoman for Denver Mayor Michael Hancock, who supports the measure.

"The taxing, licensing and regulation have all been addressed, but this was one aspect that hasn't been," she said.

The Colorado chapter of the American Civil Liberties Union was quick to blast the proposal, calling it "ill-advised, unnecessary and unconstitutional."

Mark Silverstein, the ACLU's legal director in Colorado, said when voters approved legalizing marijuana, it was under the understanding that it would be regulated like alcohol.

"No one risks a year in jail for drinking a beer in their fenced backyard, yet this ordinance would make criminals once again of persons who enjoy a legal joint on their back porch, if anyone can see or smell (it) from a public area or a nearby property," he said.

But the mayor said all the measure would do is clarify where people can consume marijuana.

"This proposed ordinance clearly communicates what our residents and visitors are and are not allowed to do in public," Hancock said in a statement. "It also ensures that our public spaces remain enjoyable for residents, families and tourists."

The proposal will be debated next week before a Denver City Council committee tasked with implementing new pot laws.

(Reporting by Keith Coffman; Editing by Dan Whitcomb and Eric Beech)