No government agency has admitted to developing insect-size spy drones but various official and private organisations have admitted that they are trying.

	A mechanical fly being developed at Harvard College

But official protestations of innocence have failed to kill speculation of government involvement after a handful of sightings of the objects at political events in New York and Washington.

Vanessa Alarcon, a university student who was working at an anti-war rally in the American capital last month, told the Washington Post: "I heard someone say, 'Oh my God, look at those.'

"I look up and I'm like, 'What the hell is that?'. They looked like dragonflies or little helicopters. But I mean, those are not insects."

Bernard Crane, a lawyer who was at the same event, said he had "never seen anything like it in my life". He added: "They were large for dragonflies. I thought, 'Is that mechanical or is that alive?'"

The incident has similarities with an alleged sighting at the 2004 Republican National Convention in New York when one peace march participant described on the internet seeing "a jet-black dragonfly hovering about 10 feet off the ground, precisely in the middle of 7th Avenue".

Entomologists suggest that the objects are indeed dragonflies. Jerry Louton, an expert at the National Museum of Natural History, said Washington was home to large, impressively-decorated dragonflies that "can knock your socks off".

However, he admitted that the dragonfly theory did not explain claims made independently by three people at the Washington event.

They all described seeing a row of spheres the size of small berries attached along the tails of the "dragonflies".

They also reported seeing at least three together. Mr Louton said that dragonflies never fly in a pack.

The CIA secretly developed a petrol-powered dragonfly drone back in the 1970s but the "insectothopter" was considered a failure as it couldn't handle crosswinds.

The CIA refused to discuss its subsequent work but it is known that the Defence Department has been funding research into inserting computer chips into moth pupae to create "cyborg moths" whose flight muscles can be controlled remotely.

Although experts say there are still considerable technical hurdles - not least finding a way of protecting the creations from hungry birds - some concede it is possible that some agency has secretly managed to make something that works.

"America can be pretty sneaky," said Tom Ehrhard, a retired Air Force colonel and expert on unmanned aerial craft.

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