Colonel George Fenton, director of the US joint non-lethal weapons office, said western values, including respect for human life, coupled with technological advances, "underscore the importance of non-lethal weapons in armed conflict".

Armed forces would face the likelihood of more frequent clashes involving civilian populations as cities got bigger, he said.

Russell Glenn of the Rand corporation also told the Jane's defence information systems conference: "An army having to fight in a modern megalopolis not only confronts a daunting task in confronting an enemy; the tasks associated with the control and support of non-combatants could easily demand more manpower than was necessary to seize entire cities in the mid-20th century."

The answer to high casualties, he said, lay in non-lethal weapons and intelligence from unmanned aerial vehicles - able to transmit images while flying between buildings. Remote-controlled robots would mount road blocks and seal routes with smoke or foam.

In a report for the European parliament, Steve Wright of the Manchester-based Omega foundation listed ultra-sound generators, which cause disorientation, vomiting and defecation; "human capture nets" laced with chemical irritant or electrified; foam guns; blinding lasers; and "thermal guns", which incapacitate through a wall by raising body temperatures to 42C.