Iran's Revolutionary Guards are telling the United States to "recount" the drones in its fleet as they insist that -- despite US denials -- they captured a small US unmanned spy plane over Gulf waters, Iranian media said Wednesday.

"Its capture is not an issue the Americans can easily refute," Guards spokesman Brigadier General Ramezan Sharif was quoted as saying.

"I advise the American commanders to recount their drones accurately," he said.

The Guards on Tuesday claimed to have recently captured a ScanEagle drone, a low-cost, short-range unmanned aircraft made by Boeing that measures 1.4 metres (4.5 feet) long and with a wingspan of three metres (10 feet).

They said the craft was seized in Iranian airspace but gave no details about how it was captured intact, nor where or when. State television showed images of what it said was the drone: a grey, unmarked vehicle suspended in a hangar.

A spokesman for the US Fifth Fleet based in the Gulf said none of its drones was missing, and a White House spokesman said there is "no evidence" the Iranian claim was true.

A year ago, Iran displayed a bigger and vastly more sophisticated US drone, a bat-winged stealth RQ-170 Sentinel, it said it had captured by hacking its guidance system.

US officials, after initially denying that Sentinel drone had been inside Iran airspace, ended up admitting it had been lost during a CIA mission, but contended it had likely suffered a malfunction that brought it down. US President Barack Obama unsuccessfully asked Iran to return it.

The ScanEagle that Iran says it now possesses is a much cheaper, simpler drone than the RQ-170 Sentinel. It is principally designed to feed back video images over a radio link to operators up to 100 kilometres (60 miles) away.

US and allied forces used ScanEagles in Iraq and Afghanistan, and several other countries operate the drone, including Australia, Canada, Poland and the United Arab Emirates, according to Boeing background information. The drone is also used for civilian purposes such as tracking fish or oil platform observation.

Sharif maintained the Guards' assertion that the drone held by Iran came from a US Navy vessel in the Gulf, but no evidence was given to support that.

Speaking to the Etemad daily, Sharif said the drone was on a reconnaissance mission hovering over Iranian military sites and oil terminals.

"We have extracted data off the drone... it shows what the Americans were looking for," Sharif said.

"The drone was gathering intelligence on military (objectives) as well as the energy sector, particularly oil transitions at terminals," he said without further elaboration.

He said "more information would be released if necessary."

Sharif also insisted "the Americans will sooner or later confirm that their drone has been captured."