The Obama administration has sought to lessen the fear of prosecution for banks doing business with licensed marijuana companies, further encouraging US states such as Colorado and Washington that are experimenting with legalising the drug.

The Justice and Treasury departments outlined the policy in writing to federal prosecutors and financial institutions nationwide, the Reuters news agency reported.

The guidance stopped short of promising immunity for banks, but made clear that criminal prosecution for money laundering and other crimes was unlikely if they met a series of conditions, officials said.

Currently, processing money from marijuana sales puts federally insured banks at risk of drug racketeering charges, and they therefore refuse to open accounts for marijuana-related businesses, the AP news agency reported.

The guidance was intended to increase the availability of banking services, such as savings and checking accounts, to marijuana shops that typically deal in cash.

Last month, Colorado became the first state to open retail outlets legally permitted to sell marijuana to adults for recreational purposes, in a system similar to what many states have long had in place for alcohol sales.

Washington state is expected to follow Colorado's lead.

The number of states approving marijuana for medical purposes has also been growing. California was the first in 1996.

It has since been followed by about 20 other states and the District of Columbia.

'Public safety'

US Attorney General Eric Holder said last month that the administration was planning ways to accommodate marijuana businesses so they would not always be dealing in cash.

"There's a public safety component to this. Huge amounts of cash, substantial amounts of cash just kind of lying around with no place for it to be appropriately deposited, is something that would worry me just from a law enforcement perspective," Holder said on January 23 at an appearance at the University of Virginia.

The American Bankers Association expressed scepticism that the guidance would make much difference.

Marijuana sales still violate federal law, so banks are still at risk, said Rob Rowe, a lawyer with the trade group.

"Compliance by a bank will still require extensive resources to monitor any of these businesses, and it's unlikely the benefits would exceed the costs," Rowe said in an email to Reuters.

"While we greatly appreciate the efforts by the Department of Justice and the Treasury's Financial Crimes Enforcement Network (FinCEN), guidance or regulation doesn't alter the underlying challenge for banks."

The guidance would not protect banks from state laws, and if a wire transfer that moved marijuana-linked money touched a state where the drug is under strict control, a bank that handled the transfer could be open to state prosecution, experts in money-laundering said.

Individual banks may have difficulty identifying which state-licensed businesses would run afoul of the federal guidance, said Peter Djinis, a former regulatory policy official with FinCEN, now in private practice in Florida.

"These complicated and vague policies continue the uncertainty that banks have in determining whether to take the risk of conducting financial transactions with otherwise legitimate marijuana businesses," Djinis said.