MARJAH, Afghanistan (Reuters/AFP) US Marines came under intense fire on Sunday after taking over a building in the heart of a Taliban stronghold targeted by a Nato-led offensive designed to put the Afghan government back in charge. Militants fleeing the assault are booby-trapping villages as they go, commanders said Sunday. Mines and militant sniper fire were slowing progress for US-led troops a day after they stormed Marjah, a town of 80,000 in the central Helmand River valley controlled for years by militants and drug traffickers. Taliban fighters unleashed automatic gunfire at Nato helicopters flying in and out of the town of Marjah, and fired on Marines at a ceremony to raise the Afghan flag over a building to mark progress in the offensive. Captain Ryan Sparks compared the intensity of the fighting to the US-led offensive against militants in the Iraqi town of Fallujah in 2004. In Fallujah, it was just as intense. But there, we started from the north and worked down to the south. In Marjah, were coming in from different locations and working towards the centre, so were taking fire from all angles, Sparks said. Unlike Fallujah, where massive US firepower demolished the city and left great bitterness against the US-backed Iraqi government, the Marjah assault aims to eliminate militants while building goodwill for Afghan forces who will take over the area. Marines in helicopters landed in Marjah district, the last big Taliban stronghold in Helmand province, on Saturday at the start of a campaign to impose government control on rebel-held areas before US forces start a planned 2011 drawdown. Twelve Afghan civilians were killed during the offensive against Taliban, as commanders said booby traps and snipers were slowing progress. NATO claimed responsibility for the deaths, which it said happened when two rockets missed their target and landed on a compound as troops came under fire in the Nad Ali district of Helmand province. Its regrettable that in the course of our joint efforts, innocent lives were lost. We extend our heartfelt sympathies and will ensure we do all we can to avoid future incidents, US General Stanley McChrystal, the Nato commander, said in a statement. Isaf commander General Stanley McChrystal conveyed his apologies to President Hamid Karzai for this unfortunate incident, it said. Afghan President Hamid Karzai expressed sadness at the incident. He had urged Nato to exercise caution during the offensive to prevent civilian casualties. Upon hearing the news, Hamid Karzai immediately ordered an investigation as he had previously ordered that the operation should be carefully done to prevent innocent civilians being killed, a statement from the Presidents Office said. The Afghan President said 10 civilians had been killed during the major US-led military offensive in Marjah. The deaths were the first reported civilian casualties since the pre-dawn launch on Saturday of Operation Mushtarak, which is testing a new US-led war strategy aimed at re-establishing government control over the area. US forces fired mortar rounds against a Taliban position on Sunday, and the militants fired a round back which landed in the Marines compound but failed to explode. The Marines responded by firing rockets at the suspected militant position. A senior Afghan army general in southern Afghanistan, Sher Mohammad Zazai, told Reuters on Sunday that between 30 to 35 insurgents had been killed since the operation in Marjah and the nearby Nad Ali districts started. Citing commanders, Taliban spokesman Qari Mohammad Yousuf said on the groups website it had launched direct attacks on Nato-led troops in several parts of Marjah and had surrounded some in one area. Bags of drugs worth hundreds of thousands of dollars had been discovered, as were sacks of chemicals capable of producing 100 pounds of explosives, said Tim Coderre, a civilian adviser to Marine officials. Natos International Security Assistance Force (Isaf) confirmed the combined forces had suffered two deaths - one British and one American - in the assault so far. Brig-Gen Larry Nicholson, commander of the Marines in southern Afghanistan, described day one of the operation as good and said a couple of thousand Marines were already in Marjah.