The role of NATO in Afghanistan needs to be expanded to end growing lawlessness outside the capital, aid agencies have warned.

In a letter to foreign secretary Jack Straw, organisations including Christian Aid, Care International and Save the Children argued that reconstruction efforts in the country were being undermined by "radical elements."

"In addition, local struggles for power, fuelled in some areas by the opium trade, are leading to a growing fragmentation of the country," the organisations said.

"While efforts to create a national army, police force and judiciary remain at an embryonic stage, the ongoing climate of impunity means that there is no protection for the individual from the arbitrary use of power.

"Growing criminality is further compounding the insecurity felt by the Afghan population; there are numerous examples of robberies, thefts and assaults even in (supposedly) one of the most secure regions, Herat."

Although NATO is already taking control of the International Security Assistance Force, which has brought relative stability to Kabul, the aid agencies called for a fresh United Nations resolution to give the organisation a greater mandate.

"We believe that NATO has the resources, personnel, structure and ability to provide these levels of security," the letter continued.

"We would urge the UK to take the lead in this and to sponsor a UN security council resolution calling for this expansion."

However NATO's deputy secretary general, Alessandro Minuto Rizzo, highlighted that no discussions to extend its mandate in Afghanistan had taken place.