Three more Afghan civilians were killed in the assault on a southern Taliban stronghold, NATO forces said Tuesday, highlighting the toll on the population from an offensive aimed at making them safer.

The deaths — in three separate incidents — come after two errant U.S. missiles struck a house on the outskirts of the town of Marjah on Sunday, killing 12 people, half of them children. Afghan officials said Monday three Taliban fighters were in the house at the time of the attack.

About 15,000 NATO and Afghan troops are taking part in the massive offensive around Marjah — the linchpin of the Taliban logistical and opium poppy smuggling network in the militant-influenced south. U.S. Marines are spearheading the assault.

As the assault aimed at breaking the Taliban stranglehold over southern Afghanistan continued, the extremist group received a blow with the news that the Taliban's top military commander has been arrested in Pakistan.

Mullah Abdul Ghani Baradar, the No. 2 behind Afghan Taliban founder Mullah Mohammad Omar and a close associate of Usama bin Laden, was captured in the port city of Karachi, U.S. and Pakistani officials said, speaking anonymously because of the sensitivity of the information. The arrest appeared to have occurred as many as 10 days ago, and it was unclear if it had had any effect on the Marjah battle.

The offensive is the biggest joint operation since the 2001 U.S.-led invasion of Afghanistan, and a major test of a retooled NATO strategy to focus on protecting civilians, rather than killing insurgents.

But in the fourth day of an assault that could take weeks, the drumbeat of gunfire and controlled detonations of planted bombs sparked fears that civilians will bear the burden of the fight.

In two of the incidents NATO confirmed Tuesday, Afghan men came toward NATO forces and ignored shouts and hand signals to stop, NATO said. Troops opened fire and killed them.

In the third incident, two Afghan men were caught in the crossfire between insurgents and NATO forces. Both were wounded and one died of his injuries despite being given medical care, NATO said.

Taliban fighters have stepped up counterattacks against Marines and Afghan soldiers in Marjah, slowing the allied advance to a crawl despite Afghan government claims the insurgents were broken and on the run.

Though NATO has only confirmed 15 civilian deaths, an Afghan human rights group said Tuesday that they have counted 19 civilians killed since the beginning of the operation. Four of those were people who were caught in the crossfire when they had to leave their homes for various reasons.

"Their neighbors tell us that the bodies are outside and they want someone to pick them up. They say they're scared if they go outside they will also be shot dead," said Ajmal Samadi, the director of Afghanistan Rights Monitor. It was unclear whether NATO or insurgent forces were to blame for the deaths, he said.

In the streets, Taliban fighters appeared to be slipping under the cover of darkness into compounds already deemed free of weapons and explosives, then opening fire on the Marines from behind U.S. lines.

Explosions could be heard around town Tuesday as Marines endeavored to push further through streets littered with bombs and booby traps.

Squads with Lima Company, 3rd Battalion, 6th Marines fanned out into compounds to search for explosive devices and insurgents, as an armored-vehicle convoy moved forward. A mine-roller leading the way continuously detonated planted bombs as it advanced.

Residents said they were scared to be seen with NATO forces.

"Don't take pictures or the Taliban will come back to kill me," Wali Mohammad told an AP reporter as Marines searched his compound.

He said he strongly suspected insurgents would return to the area as soon as the Marines moved on.

He denied that the Taliban had holed up in his house during Monday's fighting, but said they often shot at U.S. and Afghan troops from his neighbors' house.

"When they come, we try to tell them not to use our house, but they have guns so they do what they want," the poppy farmer said.

The Marines' goal for many days has been to link up with other companies that airdropped into the city Saturday, but progress has been slow.

"It's really crucial that we get through today," said Lima Company Capt. Joshua Winfrey.

Afghan President Hamid Karzai approved the assault on Marjah only after instructing NATO and Afghan commanders to be careful about harming civilians. "This operation has been done with that in mind," the top NATO commander, U.S. Gen. Stanley McChrystal, said Monday.

Despite those instructions, NATO reported its first civilian deaths Sunday, saying two U.S. rockets veered off target by up to 600 yards (meters) and slammed into a home — killing six children and six adults.

In London, Britain's top military officer, Air Chief Marshal Sir Jock Stirrup, called the missile strike a "very serious setback" to efforts to win the support of locals, who are from the same Pashtun ethnic group as the Taliban.

NATO suspended the use of the rocket system that killed the civilians following the 12 deaths, pending an investigation.

In a separate incident unrelated to the Marjah offensive, a NATO airstrike in neighboring Kandahar province killed five civilians and wounded two. NATO said in a statement they were mistakenly believed to have been planting roadside bombs.

Afghan commanders spoke optimistically about progress in Marjah, a town of about 80,000 people seen as key to securing the restive south.

"It is very weak resistance, sporadic resistance by the enemy in some villages in Marjah area," Chief of Army Staff Bismullah Mohammadi said. Other officials have said Taliban fighters were fleeing across the border and the town should soon be cleared of insurgents.

In Marjah, however, there has been little sign the Taliban are broken. Instead, small, mobile teams of insurgents have repeatedly attacked U.S. and Afghan troops with rocket, rifle and rocket-propelled grenade fire in recent days.

Taliban fighters moved close enough to the main road to fire repeatedly on columns of mine-clearing vehicles.

Allied officials have reported only two coalition deaths so far — one American and one Briton killed Saturday. There have been no reports of wounded. Afghan officials said at least 27 insurgents were killed so far in the offensive.

As long as the town remains unstable, NATO officials cannot move to the second phase — restoring Afghan government control and rushing in aid and public services to win over inhabitants who have been living under Taliban rule for years.