Seattle's first and only recreational marijuana store had to close on Friday after running out of stock in just three days since Washington became the second U.S. state to allow pot sales to adults.

Cannabis City opened in Seattle on Tuesday with at least 4.5 kg of marijuana for sale, and by close of business Thursday it was all gone. A message on the store's phone line said it would re-open on July 21.

There were widespread concerns that shortages of pot would afflict retailers this week after the state issued its first 25 licenses to outlets, under a heavily regulated and taxed system approved by voters in November 2012.

Some business owners planned to limit the amount of marijuana early customers could buy to try to make stocks last.

Amber McGowan, manager at Cannabis City, told Reuters on Thursday the store would likely not have enough inventory to stay open for all of its regularly scheduled business hours until a delivery that was due next week.

Customers limited to 6 grams

She said the shop was only able to stay open as long as it had by limiting customers to six grams per purchase, rather than the legal limit of 28 grams.

Cannabis City manager Amber McGowan takes the marijuana shop's first call, from a potential customer, days before the grand opening Wednesday. (The Associated Press)

The roll-out of recreational sales in Colorado and then Washington comes as a broader trend of liberalization and pro-pot activism takes hold in the United States.

Progress in Washington has been slow, however, with state regulators still processing more than 300 license applications, and approved growers producing only limited harvests so far.

Industry insiders say the shortages are likely to be only temporary, caused in part by the short notice many retailers had to prepare for opening, and a surge of pent-up demand.

This week, Colorado estimated that state's total marijuana demand for this year at 130 tons.

'Off the hook'

"A year from now, product is likely going to be far more available," said Sean Green, chief executive officer of Kouchlock Productions, a marijuana producer in Washington.

Another local supplier, Wow Weed, said they were trying to help the stores, but that there was only so much they could do.

Cannabis City owner James Lathrop briefly stands outside his business, with a temporary sign over the mail slot. (The Associated Press)

"We have been hearing from retailers off the hook. My voice mail is full every single day," said Wow's Susy Wilson. "It's the same people calling over and over, hoping I'll pull something out of thin air."

Frustrated consumers in Seattle, a city of some 630,000 people, made light of the shortages, with one Twitter user urging outlets to adopt a green "Pot Light" system for their windows to show they had stock - similar to the Hot Light employed by a well-known donut brand.