The US Centres for Disease Control and Prevention is investigating a "cluster" of lung illnesses that it believes may be linked to e-cigarette use after such cases were reported in 14 states.
The CDC said there was no evidence that an infectious disease was behind the illnesses and that more information was needed to determine whether they were in fact caused by e-cigarette use.
The CDC is working with health departments in Wisconsin, Illinois, California, Indiana and Minnesota on the investigation.
Since June 28, states have reported 94 possible cases of severe lung illness tied to vaping, primarily among teenagers and young adults, according to a CDC statement on Saturday.
Patients experienced coughing, shortness of breath and fatigue. Some had serious breathing difficulties that required ventilation.
In the United States, Juul Labs is the dominant e-cigarette maker.
"Like any health-related events reportedly associated with the use of vapor products, we are monitoring these reports," Juul Labs said in a statement.
"These reports reaffirm the need to keep all tobacco and nicotine products out of the hands of youth through significant regulation on access and enforcement. We also must ensure illegal products, such as counterfeit, copycat, and those that deliver controlled substances, stay out of the market and away from youth."
Australian Associated Press