A woman smokes a Juul e-cigarette in New York, Sept. 27, 2018. REUTERS/Brendan McDermid

The U.S. Food and Drug Administration said on Tuesday it seized more than a thousand pages of documents from Juul Labs related to the company's sales and marketing practices after a surprise inspection, the latest clampdown on e-cigarette companies.

The FDA said last month it was considering a ban on flavored e-cigarettes from Juul and others as the agency grapples with an "epidemic" of youth e-cigarette use that threatens to create a new generation of nicotine addicts.

Retail sales of Juul grew more than seven-fold from 2016 to 2017, according to a study by the U.S. Centers for Disease Control and Prevention (CDC). By December, Juul comprised nearly 1 in 3 e-cigarette sales nationally, it said. The CDC study of retail sales did not include online sales or sales at “vape shops.”

Juul has about 72 percent of the U.S. market for e-cigarette sales, according to Wells Fargo.

"Juuling," along with "vaping," has become a common term for e-cigarette use by teenagers on social media and at U.S. high schools. E-cigarettes vaporize a liquid that contains nicotine, the addictive stimulant that gives smokers a rush.

The inspection, completed on Friday, followed a request in April for documents that would help the agency better understand the high rates of use and appeal among youth of Juul products.

The FDA also said it conducted inspections of several of Juul's contract manufacturing units earlier this year.

"We've now released over 50,000 pages of documents to the FDA since April that support our public statements," Juul Chief Executive Officer Kevin Burns said in a statement, adding the meeting with the regulator was constructive and transparent.

"We want to be part of the solution in preventing underage use, and we believe it will take industry and regulators working together to restrict youth access".

The FDA's regulation of e-cigarettes has come under the spotlight as Juul's popularity surged.

U.S. Senators Dick Durbin and Lisa Murkowski in a letter on Tuesday urged the FDA to immediately ban child-friendly e-cigarette flavorings and restrict online sales of nicotine products.

“While it remains unclear whether e-cigarettes can actually help adults quit smoking cigarettes, it is abundantly clear that tobacco companies are developing and marketing e-cigarette flavors that appeal to, and addict, children,” the senators wrote.

In July, Durbin, a Democrat, and Murkowski, a Republican, introduced legislation that would place restrictions on e-cigarette flavorings that appeal to children like gummy bear, chocolate cupcake and tutti fruitti.

Along with the Juul phenomenon, a new wave of lower-priced Juul knockoffs are showing up at convenience stores, vape shops and online, despite an FDA rule that banned the sale of new e-cigarettes after August 2016 without regulatory approval.

Startups and major tobacco firms have launched more than a dozen new high-nicotine devices with Juul-like designs since the FDA imposed the deadline, according to a Reuters review of the companies' online advertisements, social media posts and public statements.

Shares of tobacco companies, including Philip Morris (PM.N), Altria (MO.N), British American Tobacco (BATS.L) and Imperial Brands (IMB.L), jumped after the FDA announcement.

Juul's flash-drive-like device is recharged via a USB port. The device comes with cartridges, or pods, of nicotine-based liquids in flavors like mango, creme and mint — that critics say appeal to minors.

The newest liquid blends from Juul and its imitators - called "nicotine salts" in the industry - contain a compound called benzoic acid, which lowers the pH level of the liquid.

That reduction allows users to take in more nicotine without a bitter taste, according to the U.S. Centers for Disease Control and Prevention (CDC).

(Reporting by Tamara Mathias and Ankur Banerjee in Bengaluru and Yasmeen Abutaleb in Washington; Editing by Maju Samuel and Sriraj Kalluvila)