MARJAH, Afghanistan, (AP) – Sniper teams attacked U.S. Marines and Afghan troops across the Taliban haven of Marjah, as several gun battles erupted Monday on the third day of a major offensive to seize the extremists’ southern heartland.

Multiple firefights in different locations taxed the ability of coalition forces to provide enough air support as NATO forces forged deeper into the town, moving through suspected insurgent neighborhoods, the U.S. Marines said.

In northern Marjah, an armored column came under fire from at least three separate sniper teams, slowing its progress. One of the teams came within 155 feet (50 meters) and started firing.

Troops braced for the estimated 2.5-mile (four-kilometer) march to link up with U.S. and Afghan troops who had been airdropped into the town. Small squads of Taliban snipers initiated firefights throughout the day in an attempt to draw coalition forces into a larger ambush.

The massive offensive involving some 15,000 U.S., Afghan and British troops is the biggest joint operation since the 2001 U.S.-led invasion of Afghanistan.

On Monday, Afghan military officials gave a more optimistic view of the progress being made, 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 Marjah they are very weak. There has been low resistance. Soon we will have Marjah 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.

However, the mission faced a setback on Sunday when two U.S. rockets slammed into a home outside Marjah, killing 12 civilians. NATO said Monday that the rockets missed their target by about 600 meters, or about a third of a mile. NATO had earlier said the rockets missed their target by just 300 meters.

The civilian deaths were a major blow to NATO and Afghan efforts to win the support of residents in the Marjah area, a Taliban logistical center and a base for the lucrative opium trade that finances the insurgency. Before the offensive began Saturday, Afghan President Hamid Karzai had pleaded for the Afghan and foreign commanders to be “seriously careful for the safety of civilians.”

Karzai has called for a thorough investigation into the airstrike. Differing accounts have emerged about the details.

On Monday, Afghan Interior Minister Atmar said at the briefing in Lashkar Gah that nine civilians and two or three insurgents were among those killed, suggesting that insurgents were firing at troops from a civilian home.

“The reality is this … the enemy did capture some civilians in their house and they were firing at our forces from this house. Unfortunately our forces didn’t know that civilians were living in that house,” he said.

The top NATO commander in Afghanistan, Gen. Stanley McChrystal, apologized for “this tragic loss of life” and suspended use of the sophisticated rocket system pending a thorough review.

The rockets were fired by the High Mobility Artillery Rocket System, or HIMARS, at insurgents who had attacked U.S. and Afghan forces, wounding one American and one Afghan, NATO said. However, the projectiles veered off target and blasted the home in northern Nad Ali district, which includes Marjah, NATO added.

Karzai spokesman Waheed Omar said the president “is very upset about what happened” and has been “very seriously conveying his message” of restraint “again and again.”

Inside Marjah, sporadic firefights increased by midday as small sniper teams fired at U.S. Marines before withdrawing, hoping to lure them into chasing them into a larger ambush.

“Literally every time we stand up, we take rounds,” warned one Marine over the radio.

Marines said their ability to fight back has been tightly constrained by strict new rules of engagement that make their job more difficult and dangerous. Under the rules, troops cannot fire at people unless they commit a hostile act or show hostile intent.

“I understand the reason behind it, but it’s so hard to fight a war like this,” said Lance Corp. Travis Anderson, 20, from Altoona, Iowa. “They’re using our rules of engagement against us,” he said, stating that his platoon had repeatedly seen men dropping their guns into ditches before walking away to melt among civilians.

Allied officials have reported two coalition deaths so far — one American and one Briton killed Saturday. Afghan officials said at least 27 insurgents have been killed in the offensive.

In unrelated incidents in southern Afghanistan, NATO said two service members died Sunday — one from small-arms fire and the other from a roadside bomb explosion. The international force did not disclose their nationalities, but the British defense ministry reported that a British soldier died Sunday of wounds suffered in an explosion.