OUTSKIRTS OF MARJAH, Afghanistan, (AFP/Reuters) - Taliban fighters under siege in southern Afghanistan were putting up a tough fight Saturday, military officials said, as civilian authorities geared up to take over. The number of foreign troops killed in Operation Mushtarak rose to 12 with the death of an Isaf soldier during fighting Friday as Natos biggest assault against the militants moved into its second week. Some 15,000 US-led troops from Nato and Afghanistan are taking part in the offensive against Taliban militants who have held sway over the Marjah and Nad Ali districts of Helmand province for at least two years. Commanders said they expect the military phase of the operation to last another three weeks as they strive to clear the areas of snipers and innumerable hidden bombs left behind by fleeing fighters. Nato described fighting in pockets northeast and west of Marjah as difficult, adding insurgent activity is not limited to those areas. Of 22 foreign soldiers who died in Afghanistan in the past week, 12 were in Operation Mushtarak, said Isaf spokesman Sergeant Jeff Loftin. One Afghan soldier has been killed, said Daud Ahmadi, Helmand provincial spokesman. Afghan troops account for almost one-third of the combined force in what has been billed by NATO as an Afghan-led operation. Natos International Security Assistance Force (Isaf) said a civilian was killed by troops in Nad Ali on Friday when they mistakenly thought he was running towards them with a bomb. Ahmadi said 15 civilians have been killed in Mushtarak, but it was not clear if the figure included the ISAF report. Taliban spokesman Yousuf Ahmadi denied reports they were using human shields, but took responsibility for the planting of IEDs - the greatest threat to the advance and the biggest killer of foreign troops. President Hamid Karzai, opening parliament after its winter break, lent his support to the war against Al-Qaeda and the Taliban and again proposed reintegration of Taliban foot soldiers, who have been promised jobs, land and cash. He also renewed his call for the Taliban to accept his peace proposal. The fight against terrorists and those who prevent peace in the country should continue and our people should be able to live peaceful, prosperous lives, Karzai said. With the commander of US and Nato troops in Afghanistan, General Stanley McChrystal sitting in the front row, Karzai repeated his call for civilian casualties be avoided at all costs. I once again call on Taliban and other opposition groups... to come back and take part in peace, reconstruction and developments of their country, Karzai told lawmakers. Our nation, more than anything else needs peace and reconciliation. This is our countrys prime desire and national necessity.