Photo/IllutrationSmartphone cases containing liquids that National Consumer Affairs Center of Japan said may cause burns (Ami Nomura)

Leaking liquid contents in some types of sparkling smartphone cases have burned at least nine people, the National Consumer Affairs Center of Japan said.

The NCAC issued a warning over the cases, saying: "Please avoid keeping them close to your skin for a long time. Don't leave them in the pockets of your clothes.”

Nine people reported being burned by liquids that oozed out of their smartphone cases from late April 2016 to the end of March this year, the center said.

In three cases, victims required more than a month of medical treatment to heal chemical burns. A woman in her 30s was seriously injured when liquids seeped out onto her shoulder and arm.

The NCAC investigated five brands of cases that were high ranked by online shopping sites and found all the liquids inside them were “mineral oils,” whose ingredients closely resemble kerosene.

The liquids could cause blisters and burns if they come in contact with skin for a prolonged period, the center said.