Five Afghan children have been injured, some seriously, by cannon fire from a British Apache helicopter, according to UK defence officials.

It is believed they were hit by stray bullets during an intended attack on an insurgent as they worked in a field in the Nahr-e-Saraj district of Helmand province, on Saturday.

The children were taken to Camp Bastion, the main British base in Helmand, for treatment, the Ministry of Defence said.

Officials are investigating the incident which is likely to focus on the accuracy of the Apache's cannon and the speed with which they fire.

The MoD is expected to offer compensation in the form of ex gratia payments to the families of the children.

An MoD spokeswoman said: "On 23 July, UK forces used an Apache attack helicopter to conduct a strike operation against positively identified insurgents who were seen operating in Nahr-e Saraj (south). Regrettably, five local children working in a neighbouring field were injured during the strike. The five children were taken to Isaf medical facilities before being taken on to Camp Bastion where they received medical care."

She added: "Any incident involving civilian casualties is a matter of deep regret and we take every possible measure to avoid such incidents. A shooting incident review is now under way and UK officials in Afghanistan are keeping the provincial governor abreast of the results of the investigation."

Isaf said in a statement: "We are aware of an incident in Nahr-e-Saraj where a known insurgent was targeted by a coalition helicopter. As a result of the engagement, five local children were regrettably injured. Subsequently, the injured were evacuated to an International Security Assistance Force medical facility and are currently being treated.

"Isaf is currently assessing this event, and more details will be released as they become available. Coalition forces take every allegation of civilian casualties seriously."

Isaf has said reducing civilian casualties is a priority. More than 400 civilians were killed by foreign and Afghan forces last year and over 2,000 killed by Taliban-led insurgents, according to UN figures.