LONDON: The death of 12 Afghan civilians in rocket attacks during a major US-led offensive is a “very serious setback”, the head of Britain’s armed forces admitted Monday.

But Jock Stirrup, the chief of the defence staff, said NATO forces could overcome the incident, while warning that the success

of Operation Mushtarak could not be judged for about a year.

“It is a very serious setback. It is not one which can’t be overcome and of course the Afghans themselves, the local government, play a key role in this,” Stirrup told BBC radio.

He was speaking the day after 12 Afghan civilians were killed when two rockets missed their targets and landed on a compound

as troops came under fire in the Nad Ali district of Helmand province.

US Marines are leading 15,000 US, NATO and Afghan troops in the assault focusing on Marjah town, which has been controlled by Taliban and drug traffickers for years. Stirrup urged patience in any assessment of the success of the mission.

“This a very challenging operation. Time is important and it is going to take

time for us to persuade the locals that they should be accepting the Afghan government,” he said.

“In about 12 months, we will be able to look back and say that this whole operation has been successful.”