Ride-hailing company Lyft has halted its electric bike service in San Francisco after at least two appear to have caught fire.

First, a photo of a burnt-put Lyft bikes was shared on Twitter on July 27 by Zach Rutta. The San Francisco Examiner than reported that a second Lyft bike had caught fire on July 31. Both appear to have extensive fire damage to their battery packs, which are fitted to the bike's frame and just ahead of where the rider sits.

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After the Wednesday fire, Lyft users in San Francisco noticed that bikes in the area were marked as unavailable by the mobile app.

In all, Lyft operates almost a thousand of the electric bikes under the Bay Wheels name in San Francisco and the Bay Area. It also hires out the bikes in New York City.

According to the Examiner, a witness had already extinguished the July 31 fire by the time firefighters had arrived. A fire official later said the incident had been labeled a "battery pack issue" and a "fire from battery pack."

It isn't known yet if the fires were caused by a problem with the bikes, or is vandalism is to blame in one or both incidents.

Lyft said in a statement: "Because the safety of our riders is our number one concern, we are temporarily making the e-bike fleet unavailable to riders while we investigate and update our battery technology."

The bikes, which carry Lyft branding but are operated by Bay Wheels, had only returned to San Francisco streets a fortnight earlier, following a legal battle between Lyft and the San Francisco Municipal Transportation Agency (SFMTA).

The dispute erupted when Lyft sued the city, claiming it had an exclusive contract to operate electric hire bikes in San Francisco, and that this agreement prevented rivals from offering their own e-bikes.