Garmin said Saturday that client data from the company's smartwatch and GPS services -- offline since Thursday -- has not been compromised, despite fears of a ransomware attack.
"Garmin has no indication that this outage has affected your data, including activity, payment or other personal information," the company said on its website.
It added: "Although Garmin Connect is not accessible during the outage, activity and health and wellness data collected from Garmin devices during the outage is stored on the device and will appear in Garmin Connect once the user syncs their device."
Garmin Connect is a health and fitness tracker.
The group, based in Olathe, Kansas, said it was working to restore service as quickly as possible.
While Garmin has yet to explain the nature of the outage, specialised sites have speculated that the company fell victim to a so-called ransomware attack, in which hackers block a victim's access to its own data unless a ransom is paid.
The company's Garmin Pilot application, used by pilots to plan flights, has also been blocked, as has the flyGarmin aviation database service.
The outage has affected Garmin's call centers, meaning the group is unable to respond to customers' phone calls, emails or online chats.
Specialized website ZDNet reported that after the incident began early Thursday, "several Garmin employees took to social media to share details about the attack, all calling it a ransomware attack."
It added: "ZDNet has interviewed several and confirmed their claims."