Pilotless “sniffer-drone” aircraft could soon be flying above cities searching for the first whiff of a chemical attack.

Researchers in the United States say the drones could give early warnings of terrorist gas attacks with nerve agents such as tabun, sarin or VX.

They could also provide the emergency services with rapid analysis of a situation following a chemical attack.

The drones would be fitted with a newly developed ultra-lightweight sensor system that can detect nerve gas and blister agents.