LONDON (AFP) - The death of 12 Afghan civilians in rocket attacks during a major US-led offensive is a very serious setback, the head of Britains 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 cant 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. Meanwhile, hidden bombs planted in roads in southern Afghanistan are preventing people wounded in a massive military offensive against the Taliban from reaching hospital, the Red Cross said Tuesday. We have had an increase in people coming in in the past 72 hours, we have walking wounded and the problem is to get those who need more help to where they can get it, said Bijan Farnoudi, the ICRCs communication coordinator. The road from Marjah to Lashkar Gah, 20 kilometres (12 miles) away, was blocked so wounded could not be transported from the ICRCs first aid post in Marjah to one of the two hospitals in the provincial capital, he said. The roads are littered with mines, Farnoudi told AFP, adding that troops and checkpoints were also making it almost impossible to move out of Marjah. Most people turning up at the first aid post were men, he said, adding: It is possible that a good proportion of them are fighters. Their injuries were consistent with battlefield wounds, he said. The mission against the Taliban-who have controlled the region for years in tandem with drug traffickers-is in its fourth day, and military officials said progress is being slowed by improvised bombs, known as IEDs. Afghan army commanders say the IEDs-the biggest killers of foreign troops-have been found on roads, in fields, hanging from trees and even plastered into the walls of houses. Afghan army chief of staff Besmillah Kahn said troops were being slowed in their advance by the need to clear hundreds of IEDs.