US Marines pummeled insurgents with mortars, sniper fire and missiles as fighting intensified Thursday in two areas of the Taliban southern stronghold of Marjah, where US and Afghan forces are facing stubborn resistance in an operation now in its sixth day. Marines traded machine-gun fire after coming under attack by insurgents with rocket-propelled grenades. One Marine company attacked Taliban positions surrounding them at dawn. Marines and Afghan troops continued to battle “stiff resistance” in different parts of town, a Marine spokesman said Thursday “We're seeing more fortified positions. They're standing their ground, essentially,” Lt. Josh Diddams said. “You don't know where you're going to get a little pop up of insurgents who are going to stay and fight.”The fighting in Marjah has followed a similar pattern over the past few days: relatively light in the morning with sniper fire intensifying through midday before subsiding at nightfall. But there were also pockets of calm. Families trickled back and shops reopened in a northern part of town as a small measure of normalcy returned to parts of Marjah that are under Afghan and NATO control.Their donkeys laden down with their belongings, several families could be seen coming back to their homes in a sign that some civilians believed the fighting is over in zones secured by NATO troops. Several storekeepers reopened their shops in the bullet-riddled northern bazaar in the northern part of town, as customers lined up to buy goods for the first time in nearly a week. This is the biggest offensive since the 2001 US-led invasion of Afghanistan, and a test of President Barack Obama's strategy for reversing the rise of the Taliban while protecting civilians. Five NATO service members and one Afghan soldier have been killed since the attack on Marjah, the hub of the Taliban's southern logistics and drug-smuggling network, began Saturday. About 40 insurgents have been killed, Helmand Gov. Gulab Mangal said. NATO had previously reported six deaths, but said Thursday that one death had mistakenly been reported twice. Once the town of 80,000 people is secure, NATO plans to rush in civil administrators to revive schools, health clinics and electricity in hopes of winning public support to discourage the Taliban from returning. But in a sign of the difficulty that NATO faces in trying to reverse the rise of militants, eight members of the Afghan National Police Wednesday night defected to the Taliban, a police official said Thursday. Eight policemen in Wardak province's Chak district abandoned their posts and joined with Taliban militants in the area late Wednesday, said Mirza Khan, deputy provincial police chief. Khan said one of the policemen had previous ties with the Taliban. The incident is under investigation.A Taliban spokesman called The Associated Press to confirm the defection. “These policemen came on their own and told us they want to join with the Taliban,” Zabiullah Mujahid said. “Now they are with us.” He said they came with their weapons and ammunition.