The potentially harmful health effects of heated and aerosolized constituents of e-cigarette liquid – including solvents, flavorants and toxicants – are not completely understood, said the United States Surgeon General Vivek Murthy.
According to a report published on Thursday, e-cigarette use is exploding among young people and is now "a major public health concern."
The findings have sparked disagreement from experts in the US and in Britain, where the devices are seen more favourably.
The battery-powered devices heat a liquid containing nicotine into a vapor that is inhaled, and some experts worry that a new generation of smokers is becoming addicted.
About one in six US high school students say they have used e-cigarettes in the past month.
"E-cigarette use has increased considerably in recent years, growing an astounding 900% among high school students from 2011 to 2015," said Murthy in the preface to the report.
"These products are now the most commonly used form of tobacco among youth in the United States, surpassing conventional tobacco products, including cigarettes, cigars, chewing tobacco, and hookahs."
Murthy said the dangers of e-cigarettes include nicotine, which "can cause addiction and can harm the developing adolescent brain."
For young people, nicotine use in any form is unsafe, said the report, which was written and reviewed by more than 150 experts.
It also said the secondhand aerosol that is exhaled into the air by e-cigarette users can expose others to potentially harmful chemicals.
But Murthy admitted to "gaps in scientific evidence" when it comes to the dangers of e-cigarettes.
"However, although e-cigarettes generally emit fewer toxicants than combustible tobacco products, we know that aerosol from e-cigarettes is not harmless."
He urged greater federal regulations, raising and enforcing minimum age of sale laws, and media campaigns to educate the public on the harms of e-cigarettes.
The report contained no new scientific research, but could still serve as an "important document for healthcare providers, policy makers and others", said Patricia Folan, director of the Center for Tobacco Control at Northwell Health in New York.
Experts in Britain, however, questioned the US take on e-cigarettes as a threat.
"The position in the UK is very different," said Kevin Fenton, national director of health and wellbeing at Public Health England.
"Our review of the evidence found e-cigarette use carries a fraction of the risk of smoking, a conclusion reiterated by the Royal College of Physicians earlier this year," he said.
"No new evidence has been published to contradict this."
And multiple studies have shown that e-cigarettes may help traditional smokers abandon cigarettes.
Research released earlier this year by the University College London showed that e-cigarettes may have helped some 18,000 smokers in England kick the tobacco habit last year.
Peter Hajek, director of the Tobacco Dependence Research Unit at Queen Mary University of London, described vaping as "a great public health opportunity" because it can help smokers transition away from cigarettes.
"The new US report's conclusions do not tally with what the actual data show," he added.
"It is simply not true that e-cigarettes are a tobacco product or that vaping lures children to smoking or that it creates dependence in non-smokers."
Len Horovitz, a pulmonary specialist at Lenox Hill Hospital in New York, disagreed, saying teens will transition to smoking cigarettes after using e-cigarettes.
"The gateway to cigarette smoking may still be the e-cigarette, and there is no safe level of traditional smoking."
Worldwide, smoking kills around six million people each year, mostly in low-income countries, according to the World Health Organisation.