Marjah , Afghanistan ( CNN ) -- The NATO operation aimed at tackling a Taliban stronghold in southern Afghanistan is making progress , officials say . Operation Moshtarak , in the southern city of Marjah , is going `` very smooth '' so far , U.S. Marine Corps Lt. Col. Mark Dietz told CNN Monday . He said the Marines had bypassed several lines of roadside bombs and were in the process of clearing the city `` from the inside out . '' There has been some active combat , said CNN 's Atia Abawi , who is embedded with U.S. Marines in Marjah . Watch Marines in action in Marjah . The Taliban is `` popping up in various parts of the city , '' Abawi said . `` We went just a couple hundred meters outside of the position that we 're usually based in -- we were under gunfire ourselves . '' Marjah is `` practically a ghost town right now , '' she added . `` You ca n't really find out where they 're hiding . ... They can be anywhere . '' Abawi said NATO forces are hoping to draw more `` extremist fighters '' out of their hiding places and into combat . Between two and three insurgents are being killed in each clash on average , she said , and NATO commanders `` do n't expect [ fighting ] to die down anytime soon . '' Combined forces have been `` conducting a number of mounted and dismounted patrols , '' according to a news release from the International Security Assistance Force . `` Elements of the combined force have been engaged in periodic small arms fire fights , and a number of insurgents have been either killed or detained , '' ISAF said . The release noted that the combined force `` has sustained some injuries . '' Troops recovered $ 8.7 million worth of raw opium , Abawi reported . Clearing out poppy fields is a key part of Operation Moshtarak , the biggest offensive since the Afghan war started in 2001 . The Taliban finances its activities in part through the illegal opium trade . The operation in Helmand province focuses on Marjah and surrounding areas . Marjah , known as the country 's heroin capital , is where the Taliban set up a shadow government . The city is now the `` last bastion '' for the Taliban in the Helmand River valley , Dietz said . One of the biggest challenges facing the NATO mission is attacking the Taliban while limiting civilian casualties . On Sunday , 12 civilians died in a rocket attack by coalition troops . ISAF first issued a statement saying the civilians were killed when a rocket hit about 600 meters ( 650 yards ) from its intended target . A senior ISAF official , however , later said the rocket was not off target at all . The official said troops hit the building they were aiming for but did not know that civilians were inside . The official has direct knowledge of the incident , but he declined to be named because an investigation is under way . `` It 's regrettable that in the course of our joint efforts , innocent lives were lost , '' U.S. Gen. Stanley McChrystal , top commander of U.S. forces in Afghanistan , said in a statement Sunday . McChrystal has given troops clear orders to avoid civilian casualties . `` Even if there is a Taliban fighter within a compound and [ troops ] suspect that a civilian is in there , they are not allowed to fire , '' Abawi reported . A group of Afghan villagers came to the Marines with two wounded teenage boys . The military treated them and sent them by medevac to the nearest hospital . Villagers told the troops that the Taliban were using private homes to fire at U.S. forces . In an incident not connected to Operation Moshtarak , an airstrike by NATO forces accidentally killed five Afghan civilians and wounded two Monday in Afghanistan , the military said . A joint Afghan-ISAF patrol in Kandahar province observed a group of people who seemed to be burying an improvised explosive device , ISAF said . The patrol called in an airstrike , but later found out that the group was not planting an IED . `` We regret this tragic accident and offer our sympathies to the families of those killed and injured , '' Maj. Gen. Michael Regner , ISAF Joint Command deputy chief of staff , said in the ISAF statement . About 15,000 Afghan and NATO forces are taking part in Operation Moshtarak . U.S. troops are leading the mission . `` The majority of the fighting , the majority of the headway being made , is by the U.S. forces , '' Abawi reported . She added that she had seen many Afghan soldiers `` and to be quite honest with you , they 're not ready to fight . '' More on Operation Moshtarak from Afghanistan Crossroads blog . In a statement Monday , Master Sgt . Jeff Loftin , a spokesman for the NATO-led International Security Assistance Force , said forces were in the `` clearing phase . '' Afghan and NATO troops discovered 5,500 pounds of explosives and seized a Taliban commander responsible for small arms ambushes and roadside attacks on troops , Loftin said . Forces were facing primarily small arms fire , he said . Several insurgents have been killed or detained and some coalition troop members have suffered injuries . Loftin did not specify the number of militants and troops wounded or killed . Over the weekend , provincial spokesman Dawoud Ahmadi said 27 Taliban fighters were killed . A Taliban spokesman for the Marjah area disputed the numbers , and said there had been six Taliban casualties while militants had killed 192 troops . The Taliban has often inflated casualty figures in the past . CNN 's Nic Robertson , Frederik Pleitgen , Barbara Starr and Per Nyberg and journalist Mati Matiullah contributed to this report .
NEW : Marines clearing Taliban 's `` last bastion '' in area from inside out , spokesman says . NEW : Anti-Taliban operation 's progress is `` very smooth , '' U.S. military says . NEW : Five Afghan civilians killed in airstrike not connected to Operation Moshtarak . Troops in Marjah recovered $ 8.7 million worth of raw opium , says embedded CNN reporter .