The US military is funding stealth motorcycles that would allow riders to sneak up quickly on unsuspecting enemies.

The motorcycles operate at 55 decibels, about the level of an electric toothbrush or typical conversation, and can reach 130kmh. The extreme quiet is due to the use of electric motors.

The motorcycles feature two-wheel drive to improve traction on gravel, sand or any challenging terrain. Their narrow dimensions allow them to be driven in forests or other places some military vehicles can't go.

"With a skilled rider you can get basically anywhere on the planet," said Alex Dzwill, the lead engineer for one of the motorcycles, the SilentHawk, from defence contractor Logos Technologies.

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The electric motors have limited range: The SilentHawk lasts two hours and the other bike, NightMare, has 100 kilometres of power. So the motorcycles are modified with hybrid engines that can burn almost any type of fuel, such as gasoline, jet fuel, kerosene or diesel.

"Out on a mission you're not always going to have premium gasoline from the Shell station. You'll be driving through hazardous terrains in different territories," Dzwill said.

The thinking is these hybrid engines, which are about as loud as a vacuum cleaner, can be used on the part of a mission where being quiet isn't essential. Using the back-up engine increases the SilentHawk's range to 270 kilometres.

Logos Technologies showed off a prototype of the quiet motorcycle at a defence industry trade show this week along with LSA Autonomy, maker of the Nightmare.

Both are still demonstrating and testing the technology for the Defense Advanced Research Projects Agency, which will decide if it wants to continue developing the motorcycles.

Both companies said it's too early to make any estimates about how much the motorcycles would cost once in production.

If the motorcycles are ever adapted for civilians, one possible use would be providing commuters with a quiet way to ride to work. According to Dzwill, the hybrid engine could then be turned on while the motorcycle is parked, which will recharge the battery and allow for a commute home that's no louder than brushing one's teeth.