In the weeks leading up to Halloween, pranksters dressed as ghoulish-looking clowns - some brandishing weapons and sometimes planning a robbery or other crimes - have popped up across the United States.

Since late August, the trend of trying to scare unsuspecting people has grown with scary-looking clowns lurking in the woods, appearing on dark roads or driving in cars, with some brandishing knives.

The spine-chilling sightings have been reported in several states ranging from California and Minnesota to South Carolina, New Jersey and New York, and have generated the hashtag #IfISeeAClown and @ClownSightings on Twitter, which has 335,000 followers.

Even the White House weighed in on the sightings. Press secretary Josh Earnest told reporters in response to a question at a briefing that local police take it quite seriously.

"If anything is suspicious, anything, be it somebody verbally or physically acting menacing in any type of costume, notify the police right away," said Capt. Laurence Martin of the Wayne Police Department in New Jersey, which responded to a report of a clown sighting last week.

The phenomenon has gone viral, even spreading overseas, with scary clown sightings as far away as Britain and the Netherlands. British police forces have been called to several incidents around the country involving pranksters dressed as clowns jumping out and trying to frighten people.

McDonald's says it will scale back use of its smiley-faced mascot Ronald McDonald, fearing he could be mistaken for one of the malevolent "creepy clowns" terrorizing Americans in an odd national craze.

While the reports and hoax calls have been a headache for police, a concern for parents and have also resulted in arrests in some states, it has been a boost for online costume stores. "There has been a bit of an uptick," said Leigh Wendinger, the inbound marketing manager for Minnesota online retailer HalloweenCostumes.com. She said clown costumes are up about 40 percent this year but it was difficult to say if it is due to the creepy clown sightings.

Online retailer HalloweenExpress.com has seen a three-fold rise in clown masks this year. The Kentucky-based company said eight of the top 10 sellers are evil or scary clown masks this season, compared to five in the top 10 last year.