This weapon, which is called the Active Denial System, has been developed by the Joint Non-Lethal Weapons Directorate, in collaboration with Raytheon.

ECONOMIST: Electromagnetic weapons

This device, developed at the Joint Non-Lethal Weapons Directorate in Quantico, Virginia, is a microwave transmitter the size and shape of a small satellite dish that pivots on top of an armoured car.

ECONOMIST: Electromagnetic weapons

The sonic devices could prove less deadly than so-called "non-lethal" weapons such as Tazers and pepper spray guns that have actually caused some fatalities.

NPR: Focused Sound 'Laser' for Crowd Control

Whether these weapons will be deployed in other, non-lethal ways or used primarily as deadly weapons in combat remains to be seen.

FORBES: Ray Guns Beam from H.G. Wells to Kickstarter and Beyond

After the meeting, the European Union announced changes to its arms embargo on Syria, allowing EU states to provide armoured vehicles, non-lethal military equipment and technical aid to the rebels, but not weapons.

BBC: Syria conflict: John Kerry extends US aid to rebels