Snipers, booby traps, and a maze of mines have hampered progress as U.S. Marines, along with British and Afghan troops push their way into the Taliban stronghold of Marja.
"Some units have been heavily engaged in day long firefights," Capt. Abraham Sipe of the U.S. Marines-Region Command South told ABC News.
Sipe expressed surprise at the high number of IEDs [improvised explosive devices] they had encountered. Disarming them dramatically slows down the operation.
"This is a steady, methodical process. We have to be deliberate to safeguard the lives of the Marines and their Afghan partners," Stipe said.
Five to six explosions an hour can be heard in Marja, nearly all believed to be controlled detonations of booby traps.
In a city where all the roads and buildings are made of dirt, the bombs can be buried into anything and the Taliban planted explosives in doorways, paths, windows and along canals. The invaders are using bomb sniffing dogs, minesweepers and anti-bomb devices that roll in front of vehicles to set off any bombs.
Nevertheless, after a path was cleared for an armored vehicle to reach a bridge into Marja, it struck a mine on the bridge, according to ABC News' embedded reporter. No one was hurt in the explosion.
While the Marines believe they have taken control of all the key buildings and major intersections of Marja they are not yet sure exactly which parts of the town remain hostile.
"We have achieved our initial objectives and have presence in a fair bit of the area, but we still have a large area to clear," Stipe said.
As the remaining Taliban are forced into smaller areas of the city, the amount of sniping and firefights has picked up.
"In some places, for one reason or another, they have decided to stand and fight," Stipe said.
Witness Abdul Khaliq told ABC News that he had heard frequent gunfire in the center of Marjah today and that the main fighting is in and around the market area.
Khaliq added that U.S. and British armored vehicles were having difficulty entering the town with many stalled at key bridges that are rigged with homemade bombs.
Gen. Stanley McChrystal, the top U.S. commander in Afghanistan, told ABC News he was pleased with the operation so far.
"There still remains plenty of danger and hard work ahead. But I will say how proud I am of the Afghan and coalition troops who are performing impressively well given the scale and complexity of the mission," McChrystal said.
Lt. Col. Mark Dietz, deputy commander of the Marines in Helmand, told BBC news that invading forces had met with considerably less resistance than initially anticipated.
Original estimates from intelligence indicated that there were between 400-800 insurgents holed up in the town that Dietz described as the "last bastion of the Taliban" in the Helmand valley.
"There have been several pockets of stiff resistance," Dietz said.
Marja Fight Includes Civilian Casualties
Twice since Sunday the Marines have tried to reach the bazaar area in southern Marja, and both times they have been met with heavy gunfire forcing them to call in Harrier jets and attack helicopters with Hellfire missiles.
The U.S. widely telegraphed last week that the offensive was coming, partly in the hopes of minimizing civilian casualties. Nevertheless, the military announced today that three more civilians were killed accidentally during the assault on Marja.
One man was shot when he was inside a building that the Taliban was using to fire on Afghan and allied soldiers Sunday. Although taken to a medical center, he died of his wounds.
Two other civilians died on Sunday and Monday when they ignored or didn't understand warnings, including flares and warning shots, to say away from American posts. Fearing they were suicide bombers, the men were shot when they continued to advance. One man even began running towards the Americans, the statement said.
"These incidents represent some of the most difficult situations being faced by Afghan and ISAF forces conducting Operation Mostarak. Our forces are continuing to do everything they can to protect civilians," said Col.Steven Baker, ISAF Joint Command Combined Joint Operations Cell director.
Earlier in the assault, 12 civilians,including six children, were killed in an artillery strike. McChrystal expressed his regret at this "tragic loss of life" and immediately suspended the use of the rocket launching system used.
"When President Karzai approved the conduct of this operation, he gave us some specific guidance and that guidance was to continue to protect the people of Afghanistan, so this operation has been done with that in mind," McChrystal told reporters today.
The Taliban propaganda machine immediately picked up on the incident.
"Infidels had promised that no civilians would be killed, but so for 17 civilians have been killed by America's rocket attack," they said in an online statement.
The international forces, mindful of the potential damage a high civilian death toll could bring to the outcome of Operation Moshtarak, say they are doing everything they can to avoid such a thing.
"I have spoken to President Karzai and I wanted him to know that we are doing everything in our power to minimize civilian casualties," Prime Minister Gordon Brown said in a statement today.
ABC News witnessed Marines giving medical assistance to a man and young girl who had been shot after being caught in a crossfire. The girl was hit in the knee and the man in the elbow. Both were flown by helicopter to a U.S .medical facility.
"Engagement with locals is absolutely vital," Dietz told the BBC.
Some here say they welcome the change U.S. and Afghan forces could bring.
"I'm happy that the Marines and Afghan forces are here because my kids will be able to go to school now," Abdul Ranee, a Marja resident, said. He said neither he nor his brother, cousin or children had ever been to school. His son, when asked what he wanted to be when he grew up, smiled and said he just wanted to learn to read and write.
A pharmacist approached Marines to warn them that his shop was boobytrapped with mines and asked the U.S. to get rid of them.
The offensive has been flagged for weeks to persuade Taliban fighters to leave so the area can be recaptured with minimal damage or loss of civilian life, in the hope that the roughly 100,000 people there will welcome the Afghan administration.
The attack started on Saturday with waves of helicopters ferrying troops into Marja and nearby Nad Ali district. The British troops in Nad Ali had made far more progress than expected, according to the BBC which has a reporter embedded with them.
Afghan Military Officials Upbeat
Afghan military officials gave a more optimistic view of the progress being made today, with Brig. Gen. Sher Mohammad Zazai saying Afghan and NATO forces have largely contained the insurgents and succeeded in gaining trust from residents, who have pointed out mine locations.
"Today there is no major movement of the enemy. South of Marja they are very weak. There has been low resistance. Soon we will have Marja cleared of enemies," Zazai said at a briefing in Lashkar Gah, the provincial capital of Helmand province. He added that only three Afghan troops had been injured.
Dietz was also confident of the operation's success. "In the next several days we will achieve our objective and full control of the city," he said. But Dietz conceded that it may take several weeks to flush out the die-hard fighters gone to ground in Marja's surrounding countryside.
Miguel Marquez, Mike Gudgell, Fasal Rasheed, the Associated Press and Reuters contributed to this report.