Some warned reporters not to plug them in to their laptops, as USB devices can carry malware.

The fans were part of a gift bag including a branded water bottle and a local guidebook.

Temperatures reached 33C in Singapore during the meeting.

Dutch journalist Harald Doornbos tweeted a picture of the fan.

The tweet reads: "Handy. In the press kit for the #KimTrumpSummit, there is a mini USB fan - convenient to stay cool while writing. It is pretty hot here in Singapore, 33C or so. But it does not reach Dubai, king of the oven."

But cyber-security expert Prof Alan Woodward, from Surrey University, said: "For years now, engineering people to plug in a USB stick you supplied has been a classic way of circumventing security measures to get your software on their machine.

"There's an adage in cyber-security: if you give someone physical access to your computer, it's no longer your computer. Use an unknown USB stick and you are doing just that."

The gift packs were assembled by the Ministry of Communications and Information (MCI) in Singapore.

In response, it told the BBC: "The USB fans were part of Sentosa Development Corporation's (SDC) ready stock of collaterals, originally meant for Sentosa Islander members. SDC had assessed USB fans to be a handy and thoughtful gift for the media, who would be working in Singapore's tropical climate.

"MCI and SDC have confirmed that the USB fans are simple devices with no storage or processing capabilities."