MARJAH (AFP) - Thousands of US-led troops fighting to capture a key Taliban bastion in Afghanistan risked becoming bogged down Tuesday, running into pockets of resistance and scores of buried bombs. We are advancing slowly because areas have been mined, Afghan army chief of staff Besmillah Khan said on the fourth day of the massive offensive on Marjah in the opium heartland of southern Afghanistan. A massive force of 15,000 Afghan, US and NATO troops are taking part in Operation Mushtarak, seeking to drive out militants and allow the Western-backed Afghan government to re-establish control. Death tolls are impossible to confirm independently, but officials have said that 30 Taliban, two NATO soldiers and at least 12 Afghan civilians have been killed in the Marjah battle. US and Afghan military officials said remote-controlled bombs were hampering the progress of the assault in the southern province of Helmand, an area controlled for years by militants and drug lords. The bombs, the main weapon in the Taliban arsenal and the principal killer of foreign troops in Afghanistan, are being discovered in their hundreds, Khan told reporters. Hundreds of mines have been discovered in different areas, Khan said, referring to IEDs. A spokesman for the US Marines said troops have been surprised by the number of IEDs found during their advance on Marjah, a cluster of villages with a population of about 80,000. We are definitely finding more than we expected, said Lieutenant Josh Diddams, of Taskforce Leatherneck, adding: Its a slow process. In some pockets Taliban fighters stood their ground and fought, or used guerrilla-style hit-and-run tactics, firing on combined troops from residential compounds or mosques, he said. Brigadier General Larry Nicholson, who commands the Marines in southern Afghanistan, expected the operation to last for 30 days, Diddams said. On Monday, a senior Afghan commander had said that coalition forces controlled almost all of Marjah and the district of Nad Ali, and that the Taliban had left. A NATO air strike elsewhere in Helmand killed a Taliban commander known as Sarraj-Uddin and said to have coordinated foreigners fighting for the militia, and four Arab fighters, the provincial government said. Since the assault began, NATO has acknowledged the deaths of 20 civilians - 12 when two rockets missed their target, three in separate incidents and five killed accidentally in an air strike in neighbouring Kandahar. Afghan officials said three of those killed in the rocket strike were militants. Two NATO soldiers have been killed in Mushtarak, while another six British and American soldiers have died elsewhere since the operation was launched.