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.

Prime Minister Gordon Brown said he had spoken to Afghan President Hamid Karzai to reassure him British troops were doing everything possible to minimise casualties.

"I have spoken to President Karzai and I wanted him to know that we are doing everything in our power to minimise civilian casualties," he said.

"Obviously hundreds of troops have moved into an area where we have had problems with Taliban insurgents and our forces are at risk from explosive devices."

The comments came 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."